Here is Riot's Champion spotlight for Varus. Play Video
Tell's you a little about Varus to get familiar with his abilities.

Pros

Ranged AD Carry
Good CC
Easy to farm with
Possibly the best harasser in the game

Cons :(

Squishy
Useless to your teem without farm
No Escapes

Greater Mark of Desolation: Armor Pen is really good here, but you can also use flat ad runes, but ad is easier to get in items than armor pen, so I prefer the armor pen runes.
Greater Seal of Armor: Flat Armor seals let you take the harass from the opposing ad carry a little better early on, and lets you keep that one extra life you need to stay alive when your getting ganked
Great Glyph of Scaling Magic Resist: This is the only rune I take that is not flat. The Reason? I am not going against the AP Carry so I don't really need any MR early game, but late game it is really useful.
Greater Quintessence of Attack Damage: Flat AD Quint, gives you the AD you need to kill your enemies early game.

The only thing I would maybe change on the runes are the marks, I might mix a few flat ad runes in there, or swap it to all flat ad, but the reason why is if you have that extra ad and it lets you kill the opposing ad carry early game and shut them down, then it was more than worth it.

Flash: The best summoner spell ever! It will save you SOOOO many times just in one game if you use it right.....or maybe not if your amazing at positioning.....anyway it is also good for picking up the kill when they almost got away. But I think this is a must for Varus since he has no escapes.



Ignite: Good for picking up the kill without having to turret dive, or in 1v1's when it you don't know who is going to win, I have picked up so many kills with ignite that I would not have if I did not take it, even if they kill you, your ignite may still kill them. Do not use ignite at the start of the fight, wait until they are low health!

Other Useful Spells!

Ghost: You can switch out flash for ghost, I wouldn't recommend it, but it is a possible switch.



Heal: You can switch out ignite for heal. It is kind of the same premise, except instead of picking up the kill with ignite you save yourself, then hopefully get the kill. Can be used to bait people to tower dive you, or just come back to try to last hit you when they are low, then you pop heal, they don't kill you, but you kill them, then they rage in chat! :D



Cleanse: A very good CC remover, if you are in ranked you may want to pick it if you are going up against an ashe, sona, or another champion with really good cc. Cleanse replaces ignite, it can keep you from getting killed, it just isn't my favorite option, usually it doesn't help a lot unless you are getting ganked.

Passive: Living Vengeance

Passive is AWESOME. Helps you to farm really easily after you have bought a few items, and gives you some pretty good attack speed in team fights without even having any attack speed items.

Q: Piercing Arrow

This is what makes Varus one of the best harassing ADC's in the League. But it is useless if you do not hit it. Do not use it early game to farm, try to save it to last hit, and occasionally harass, other wise you won't have enough mana to use it to pick up that kill.

W: Blighted Quiver

This will help you farm more easily early with the little extra damage, and help you finish up kills with your q.

E: Hail Of Arrows

I don't see this skill terribly useful, it slows a little which can help in chases, but it doesn't do a lot of damage. I find it best to use right after ulting, or someone else uses their CC on them, because they are stunned then slowed while trying to run away.

R: Chain Of Corruption

Great, Great, Great! For team fights, it can really make your team win a team fight, and in turn the game. I say if your team is losing the team fight, use it near the beginning of it, if they are winning it, use the ult towards the end right before then enemy team will be trying to run away.