Search around for computer assembly guides. Computers these days are pretty simple to assemble. You'll likely waste more time dealing with software issues.
Also, you might want to follow those guides loosely. It would suck to assemble a 1 gpu rig and then realize you want a rig with more gpus in it.
You'll want to make sure to grab an i7 with at least 32GB of ram
You didn't even ask the guy what his plans are for the rig. You trolling?