If you do a
Google search on something like
"build you own" motherboard cpu you will find loads of sites that take you through what components you need.
You will also find explanations on what needs to match (eg PSU must be compatible with Motherboard, Motherboard must be compatible with both CPU and RAM, etc).
Haven't posted direct links to any here as, they are all invariably online PC shops who want you to buy your new bits and pieces from them!
RC