There are several leagues to try. The owner of this site runs pinewoodderbyracing.com which runs a format more similar to head's up drag racing than a traditional pinewood derby. Three classes to pick from- BASX, MA Xtreme, and Unlimited. BASX is probably the easiest to build for, building an MA Xtreme would give you the option of running the car at the Mid America Derby as well as the leagues. BASX is a body and four 2.4 gram wheels, MA Xtreme allows for clear wheel shields, fenders and any weight wide wheel with all lettering and tread bumps visible.
There's also the NPWDRL out in Utah. 12 classes there ranging from Box Stock to Unlimited. Rules for each class are at npwdrl.com or on their message board at npwdrl.boards.net . They run a BASX Amateur class there for the new guys.
In Ohio, you'll find the Association of Pinewood Racers, apr.boards.net , 11 classes over there, including Pure Stock (stock wheelbase only) and Mini Cooper (5 oz but only 4" long).
I haven't seen any action over at the PDDR site in a while, not sure what the schedule is at that venue. pddr.proboards.com
If you're anywhere near Rosemont, MN, there's the PWDR, pwdracing.proboards.com , although I heard they closed the race shop but I haven't heard if they found a new location yet.
Did I miss anyone, guys?