How do I go about choosing the correct fan/heatsink for my processor. what information do i need to know? like fan size, etc.

120mm is pretty much the standard fan size now. Anything smaller is too noisy. All you need to know is which coolers are the best! From there it just depends on your budget and taste. Coolers that stand out in my mind include the CoolerMaster Hyper 212+, Prolimatech Megahalems, Corsair H50/H60/H70, Thermalright Venomous X, and the Noctual NH-D14.Of course you will have to make sure whatever cooler you get is compatible with your socket. If you're building a sandy bridge system, that's socket 1155. All of the coolers I mentioned will be compatible. Regarding cooler compatibility, 1156 is exactly the same as 1155. Just don't go trying to put an 1156 chip into a 1155 socket, though!