I was also in your situation, worked through the tutorials and gained proficiency enough to teach students the basics. I worked through the tutorials that came with Inventor. Basically you need to learn 2d sketching and 2d constraints. Then, learn the 3d tools to create 3d parts from the sketches. Then, bring parts into an assembly and constrain them. FYI, constraints are rules imposed that define how lines in 2d drawings and 3d parts relate to one another - parallel, perpindicular, tangent, etc. Finally, 3d parts and assemblies can be put into drawings that will communicate information to machinists or assemblers. There is much more that Inventor has to offer but the aforementioned skill are prerequisite to learning more. I find Inventor to be challenging and a lot of fun!
Here are a couple of links:
www.tsof.edu.au/resources/ cad/autocad/tutorials/tutorials.asp
www.usfirst.org/robotics/2002/ Workshops/Inventor_Tutorial_Xcat.pdf
Just Google "Inventor tutorials". Unfortunately, many of the results will assume basic knowledge of Inventor. Another good site is:
www.sdotson.com
Lots of tutorials, although more advanced and lots of links. Just ask any questions on this forum. While I am not an expert, I will be happy to help if I can.
Regards,
Henry