There are some programs out there to help you design woodworking projects using Google Sketchup. However, before you go out and buy such programs, you should check out this podcast.
Hardwood podcasts puts out a podcast Sketchup: A 3D Toolbox and it is free. It breaks down all the tools built into the program, helping you to understand what the program can do from the ground up. With anything new, you need to learn the basics first.
The podcast is not geared toward woodworking, but it will teach you the program. This is how I learned the program, and I highly recommend it. I am able to apply the knowledge from this podcast to create any woodworking project I can imagine.
I subscribed to the podcast on iTunes. This allows me to go back, when needed to review certain aspects or tools of the program. I believe you can still download it on iTunes.