For me, Autodesk Inventor has been difficult to learn due to having no formal training with any CAD software. I used youtube and books from my school library to figure most of it out. I hope that this helps learning this software easier for you than it was for me.
Step 1: Required Materials:
A copy of Autodesk Inventor 2012 -- This is free for college students!
If you are not a student, Autodesk 123D is free for everyone.
A 3D Printer -- maybe a MakerBot?
If you don't have a 3D printer a CNC would also work.
A length of 1/4 inch threaded rod for the wheels.
8 1/4 inch nuts for the wheels.
Although Autodesk Inventor is arguably the best software to design models with, it is by no means easy for everyone without cad experience. You may want to breakout your library card, log on to youtube, or sent me a comment if there is a step you don't understood/I didn't explain well enough.