Fiddlesticks is a magic based assassin with massive AOE damage and ridiculous amounts of cc. If you can get good at this guy, you will have no problem stomping every game. Just keep scrolling down and reading to learn all the ins and outs of this champion and how to use him to climb ELO like never before.

PROS

fast clears
takes little damage while clearing
high cc
high damage
AOE
difficult to shut down completely
just needs a few items to really stomp

CONS

ridiculously squishy
blue buff dependent
very low mobility
two channels, meaning he can be completely shut down against opponents with displacement
can have some problems keeping enemies within his AOE

As you are jungling, machete is a basic need in the current meta. Most junglers start with a machete and 5 health pots, but as Fiddlesticks you Drain will keep you at high hp for your entire clear (except your first one). I like to take a pink ward because it grants permanent vision and vision of wards. It can be placed in the river to thwart the opposing jungler's ganks, or just in their jungle to give your team more vision.
You definitely want to rush Spectral Wraith first, as this item gives you gold on clearing camps, AP, and spell vamp. This last stat is the main reason I take Spectral Wraith, as spell vamp will stack with your drain, meaning you will heal even more ridiculously with Drain than ever before. I'm not kidding, at level 7 with spectral wraith I've gone from 10 hp to full from one camp. And spell vamp also stacks on your ultimate's damage.
Next, your first priority should be to get your hands on a Zhonya's, because this item will ensure you get off your ultimate in every team fight. Not only does the active make you completely invulnerable for 2 seconds, but it grants a bunch of armor and AP. The final reason to rush this item is that you should be finishing it around the time the laning phase ends, about 15-20 minutes. Once you get your hands on this item, you can use it to win every team fight, as well as dive without a worry.
Your next big item should be Rabadon's for all the obvious reasons. It will not only give a 120 AP on its own, but its passive increases your AP by 30%, meaning that it actually will give you 156 AP, not counting its buff to all your other items. This item is also great to pick up because it will give you a massive power spike. Buying this item will shove your AP from around 300 or so to between 500 and 600. This will make you a terror (literally) on the Rift.
Your next items are situational, meaning they depend upon the flow of the game. You won't always want to build the same exact thing on Fiddlesticks, although of course there are items that benefit him more than others. My personal choice for the next item is Rylai's for a few reasons:

grants hp, making you harder to focus down
grants AP, buffing all your damage
slows, allowing you to more easily keep enemies within your ultimate's radius.

This last effect is the main reason why I take Rylai's on Fiddlesticks. One of Fiddlesticks' biggest weaknesses is opponents with high mobility because they can escape his ultimate and CC. This item will allow you to slow down opponents and lets you keep them close enough to shred them down.
Another item you might take is void staff. This item is really great on any AP carry because it gives plenty of AP as well as tons of magic pen. Pick this up if they are stacking MR against you (as they likely will if you're fed.) You might want to pick this up even if they aren't stacking MR anyway, because it will exponentially increase your damage.
Abyssal sceptre is a good item for if they enemy has a strong AP carry or just lots of magic damage in general, or if your team has lots of magic damage. It grants lots of MR, some AP, and it debuffs surrounding enemies' magic resist, making them even easier to wipe off the rift.
GA is a good pick if it's late game and about even and an upcoming team fight could decide the game. As Fiddlesticks you shouldn't be dying in fights because of your Zhonya and all your spell vamp. However, if you need some insurance, pick a GA up by all means. Better safe than sorry.
Banshee's veil is a good one against strong magic damage dealers. It also grants a handy spell shield to fend off any time the opponents might get a pick on you.

DREAD (passive)

This passive is pretty self explanatory and simple. It's essentially a built in Abyssal Sceptre that scales with your level.

TERRIFY (Q)

This ability is a big reason why Fiddlesticks is so feared (ha). It fears the target unit, causing them to slowly run away from you. They cannot control themselves during this time. This is your main ganking tool. Use it to follow up on your laner's CC with some of your own. The longer you can lock down your opponent, the better. This is your bread and butter in fights, as when used correctly you can lock down the opposing carries for an extremely long time. The only downside is that the feared unit runs away from you, so if you use it you will have to use some of the duration running after them, which can cause problems with your ultimate. However, this is also an upside. Let's say you come in from behind the enemy bottom lane. You fear the enemy ADC from behind, so he runs towards your lane. At this point he is a sitting duck for your laners to CC and burst down while the enemy support can only stand there uselessly. Pretty handy, especially since at level 5 just hitting the Q button will lock an opponent down for 2.25 seconds.

DRAIN (W)

This ability is Fiddlesticks' trademark, dealing damage to the enemy every second as well as healing himself. This is a tethered ability, so if the enemy gets far away enough the drain will stop, much like Nocturne's fear or Leblanc's chain. The difference is, Fiddlesticks' tether has a ridiculous range. This ability is good to use on a low opponent as they start to run away, as most people are surprised at how far this ability will reach, and you can usually pick off a fleeing carry. The downside is, this is a channeled ability, meaning if you are displaced or hit with any other hard CC, the drain will stop. Keep this in mind, as certain opponents like Lee Sin or Volibear can be devastating against Fids. The other problem is you cannot use other abilities while draining, so if they start to get away and you fear them to slow them down, your drain will be interrupted. This ability has many applications. At level 7, after picking up your second blue buff, if you max this ability you can solo dragon no problem. Just be sure that you ward and check for enemy wards because if you get caught this early in the game there isn't much you can do. I believe that if you turn on this ability before activating Zhonya's active, it will continue to drain, which is pretty funny because you're basically Swain at that point. This ability will do most of your damage and is your only way to follow up CC in ganks, not counting your ultimate.

DARK WIND (E)

This ability is another decently ranged CC ability. Not only does it silence for a respectable amount of time, but it bounces between enemies, meaning you basically have AOE silence in teamfights, which doesn't sound as devastating as it is. If your team engages first, and you manage to get in this silence quickly on their Leona or Amumu or what have you, that opposing engager won't be able to use their cc on your team for another few seconds. This means that nobody can stop you from walking all over their entire team while your team jumps on top of their carries. This ability also contributes to Fiddlesticks' ridiculous clear speeds. When clearing wraiths or wolves, throwing this ability into your drain will clear the camp in about a second once you get items. This ability is also important in ganks, because silence also stops summoner spells, like Flash.

CROWSTORM (R)

This ability. This ultimate. This ultimate is one of the strongest team fight abilities in the game. I'm not joking. It's 45% AP scaling isn't the highest out there, but with a base damage of 325 a second at level 3, you can easily destroy an entire team fight on your own when this ability is utilized correctly. I have a lot to say on this ulti, so bear with me.
First of all, this ability is hilarious for ganks. If you are the purple team and your bot lane is getting pushed into, just stand just below your team's bot lane tri brush and hit R over the wall. Nobody ever thinks to ward that specific brush, so standing there and waiting for the opportune moment to jump in is guaranteed to take at least a Flash. Combine this ability with your fear to guarantee kills on a single target. This ability is great because, just like Zac's Elastic Slingshot, it lets you get creative and unpredictable with where you gank from. Once you get this ability you don't have to walk in from the side of the lane like any old jungler. No, you pop out of nowhere and shred their health bars to nothing before they even have a chance to react.
This ability's main application is, of course, in team fights. The biggest reason why this is so strong, though, is that it can be used as an engage OR a counter engage. What I mean is, you can use it as you would expect, to jump on top of the enemy team with huge AOE like any other AOE caster, but this ability is also quite potent when used after the main engage and can turn the tide of a team fight so easily you'll wonder why more people don't play Fiddles. Let's say your team gets caught. Being the carry, you blow your flash to stay out of the fight. Your team dies without you, leaving you alone, 1v4 or so, assuming your team got one or two of the opponents. Now all you have to do if find a bush, wait until you see them chasing, hit R, and laugh as you kill them all. It really is that easy. If you're far enough ahead you really don't even need a team to fight with you, as your ulti into zhonya's will be enough guaranteed damage to get the enemy on the ropes. Now, the drawback with this ability is, it's also channeled for a few seconds before casting, so if you get caught by the enemy Lee or Tristana you will be completely negated, plus the ability will go on full cooldown. Another thing to keep in mind is that opponents need to be inside the area of effect for the damage to proc, so you need to keep moving with this on, unless the teamfight is engaging. So the main tips on this ability are:

Be creative where you use it from. Surprise attacks always do better.
Remember Zhonya
Keep moving (if it's safe)
Use CC to keep enemies in range
Also can be used as an escape, but takes a few seconds to channel--only use if flash isn't available
Avoid displacement as it will interrupt your channel

I hope you guys enjoyed this guide, it is my first so I would really appreciate comments with constructive criticism and tips to improve my guides. My goal is to be as helpful as possible to as many people as possible, so go ahead and let me know how I'm doing.