For a first project, you might want to copy a tried-and-true design, from a plan. Of course you can design your own too (much cooler)--just study the beginning of Melvyn's book very carefully. He explains how to figure out a neck angle, lay out string spacing, etc.
If you decide to design your own, get your bridge first and base everything around that. If you mess that up, the strings may lay flat against the frets or else be way too high.
You can get poplar from Lowe's. Just laminate the body from 4 or 6 pieces. It'll probably cost about $20.