Hi! I'm Alfcam, lvl 21, and I play on the EUW servers. I know that I'm only lvl 21, and that this guide will be far from perfect, so I'd love positive comments telling me how to improve this guide, not comments that say 'OMG U R N00B', you know what I mean.
I started playing Heimerdinger at around lvl 10, when I was looking for a fun champion to play, and I saw Heimerdinger, so I consulted my friends about him, and what they said was basically if you want to have fun, play Heimerdinger, if you want to win, play someone else. So I took their advice and bought Heimerdinger, and found out that he doesn't suck balls at all! Maybe (probably) that is because of the low lvl people I play against, but still. Anyhow, I used a guide which name I forgot, and which I cannot find again in the depths of Mobafire, and after the Heimer rework and the mastery rework, the only thing that still remains in my current Heimer build are the items, and because my build changed so much, I decided to make a guide of my own.

This is the order I generally put my abilities in, it is important to start with your turret, because it is your main farming tool. The reason I upgrade the 'nade so late(it rhymes! Kinda...) is because I mainly use it for the stun, not for the damage. Of course you can unlock them in another order.

I don't actually have Heimerdinger runes yet, because when I dinged 20 I bought Gangplank runes, so I'm no use here yet, but it displays the runes I think would be useful on The Heimer.

Pros:
*One of the best farmers in the game.
*Very good harass with rockets.
*When dived while in a turretnest, free kill!

Cons:
*Very squishy.
*No escapes.
*Very slow movement speed.

This is the important part of this guide.
Buy your items quickly and get to your lane quickly so you can place your first turret, and keep placing them untill you have all 3 turrets on the field. This way, you will have placed all your turrets before the minion wave reaches you, and you will be able to start farming right away.
Heimer, when harrassing with his rockets and grenades, is extremely mana hungry, so buy Tear of the Goddess first. Normally I'd go back to base when I have enough for both Boots of Speed and Tear, but if everything is going well, I'm taking almost no damage and am outharassing the enemy midlaner, I go back when I have ~3k gold. If you manage to land your grenade and have a clear view of the enemy mid laner, ult your rockets and kill the fool. If that doesn't kill the bugger ignite him/her and you have a kill! Do this untill the end of the laning phase, and you will have a couple of kills and ~200 CS. An useful trick to get some extra kills(=gold) is to position yourself in the escape route of enemies being brought low by your allies, ult grenade them and finish them off with your rockets. At this point basically nobody can stop you as you can easily kill everybody who comes near you.