Take a class. If you try to learn it yourself, you will miss a lot of features and shortcuts that make AutoCAD powerful. If you learn from a coworker, you will likely miss whatever they don't know (especially if they learned on an older version).
I do very basic editing of existing drawings, getting by with a class I had in high school over 10 years ago. I can use snaps (end point, center, tangent, intersection, etc), trim, extend, setup layers, and create dimensions. If that is all you have to do, you should be ok without a class.
If someone hasn't already setup the scaling and model space vs view space stuff, though, I can screw a drawing up pretty good.
