Before you get started, figure out your measurements. This is a good resource to help you get started and make sure to allow for the height of the keyboard you plan to be using. You might want your measurements on the low side as you can always raise your keyboard/mouse with some kind of riser. Cutting down your desk afterward is a bit tougher, which I found out the hard way. I use a books and magazines to fine tune the finished heights.
I bought all of these at Home Depot for about $80.
3 - 8' 2x2
1 - 4' x 24" top
1 - 1" 4'x8' sheet of nice plywood
Follow the pictures and image notes to walk through the process of building this desk.