basically, grab whatever lumber ya can. especially 4x4's. you can find scraps all over the place. we used to set up stuff in a friend's yard when skateboarding. same idea. just find a place where no one will mess with it if ya can.

as far as structure, just build up different heights, like long steps, with the 4x4's and attach with 2x6's or 2x8's or a series of left over 2x4's as far as you can. try to make the sections about 2 feet longer (or more) than your bike so you can roll up to em and still trick of em with a little room. use wood screws so you can take stuff apart and readjust or add on if it isn't werking.

my rig isn't impresssive, but you get the idea. good luck!