well, for starters, you gotta have in mind what kind of processor you want to have, what kind of motherboard you want (that matches the processor) and a case in which the motherboard has enough room to fit in. then you get all the memory stuff and graphics card, etc.
The core components you have to look at is the CPU, motherboard,and then memory. Forget everything else for now. Make sure these three parts go with each other in terms of speed, size, and whether it will support AMD or Intel.