I second Cory's recommendation: do the Inventor tutorials. After doing the tutorials, grab a simple object and model it. Something like a pencil or screw.
Besides Autodesk's, check out the tutorials on
Sean Dotson's excellent tutorials page. He goes into a lot more detail and covers everything from beginner through expert stuff (like iParts, animation, etc.).
If you get confused or stumped, post here - someone is sure to have an answer.
Enjoy! (designing in Inventor is one of the more fun things I do at work)
Pete