Heimerdinger is an ranged AP carry.
Heimerdinger is one of the best tactical-late game character because of his turrets which involve a very high amount of damage in time.

DONT BUYY MEKI PENDANT AS FIRST ITEM !!!
Take as first item the Doran's Ring because it gives you HP,AP,and mana regen.
Meki Pendant its the next item after the Sorceress's Boots,because you'll need The Archangell's Staff for a very large amount of mana and AP.
Archangel's Staff is a very good item for Heierdinger,he has short cooldowns,so he can use his spells very fast,to farm his item.
His very short cooldowns also help him,when he will have Will of the Ancients which will heal him on casting spells and putting his turrets.
Rod of Ages and Rylai's Crystal Scepter will make him very annoying as he will be very powerful,but also very ressistant agains enemy spells.

Maximize his turrets if you are a skilled and smart player,cuz this is Heimerdinger's strongest weapon.
Maximize the rockets after if you are not a very future-positon skilled player.
If you are skilled on future-positioning then maximize the Grenade.

For the minion waves place your turrets behind your minion waves,so they wont tank for your minions.
After the turrets are reaching rank 5,if you are on mid and you are doing it well,then place the 2 turrets at the Ghost camp near yur lane.After one turret is destroied then place an other one in his place,ensuring HP and XP for a long time,whitout your pressence to be needed.

After you will done this build you will have a very large amont of AP.
Put two turrets one close to the other then let them to clear 50% hp of the enemy champon along with the rockets,then use yout Grenade to stun the enemy near your turrets,use your UPGRADE!!!,then enjoy another kill.
When you are ganked try using your grenade to stun the target and escape without being hitted.
In tamfightplace very fast your turrets use your UPGRADE!!! and shot continiousely rockets and grenades.Use your grenades on taqrgets with low HP.