Learning a 3d CAD Software is definitely a learning curve. It may appear difficult at first, and I'm sure you gape at some of the CAD you've seen before. I still do.
However, I can assure you it's not nearly as difficult as it may appear. When opening Inventor for the first time, I suggest, as many others here already have, gravitating towards the built in help tutorials on the Ribbon. That was my very first introduction to CAD, and while some may seem somewhat daunting, they really do help. After that, you're best of experimenting on your own for a while.
It is
impossible to learn to CAD in 3d without practicing, practicing, practicing until you can't think of another object to work on. I very much suggest purchasing a set of
dial calipers. They will make your experience MUCH easier and your work infinitely more accurate.
From this point, I press you to check out
Rob Cohee's youtube channel. It is very lucky that Autodesk has a specific "Technical Evangelist" whose sole job is to provide tutorials on the software (meanwhile promoting it, of course). I will warn you that by this point, he has such a massive collection of videos that finding his core tutorials may take a while. Thankfully, youtube provides "folders" for their videos.
All in all, I really hope you follow through with your learning Inventor. It can be very rewarding and, though you may never have imagined, addictive. I can tell you from my experience that after spending one summer learning Inventor (with no prior background), I was promoted to CAD captain on team 1836 (though this is hardly impressive, the previous one graduated). That being said, it entitled me to make perhaps the greatest number of design decisions of anyone of the team, second to the team Captain. That is very rewarding.
Also, feel free to contact me at
msimon@msedesign.net at any time for help. I am very willing to help people work through their Inventor problems.
Good Luck!