Since the motherboard determines everything else about your computer, you'll need to decide what motherboard you want. You can do this by deciding the CPU and the motherboard together, then look at the details of the motherboard.
Once you've picked out the motherboard, look at the worksheet, cross reference all the specifications to see which parts will fit on your motherboard. It sounds difficult, but after downloading the parts worksheet you'll see there really isn't a whole lot to it.
TIP! After you know how to make sure the pieces fit together, you can use the same method I use, simply leaving the motherboard open in another browser windows, then selecting the parts as I go down the list.
As you can see by this table, to pick out RAM, you'll need to know the memory type, supported memory, number of pins, number of slots, the maximum memory supported and the max memory per slot. Click on the table above to download a printable version of this table with the far right colum left blank for your specifics.
I like to find all the specifications I need from any online retailer.