AutoCAD would be a great option to start if you know how to design in 2D. You can easily learn the tools and grow from there. It will give you a satisfying sense of accomplishment, which is good. But it won't last long. The more you use it, the more you begin questioning yourself "do I really need to go through all of these steps in order to achieve X?" and the answer will be yes... as long as you don't take a peek at what the real 3D packages can do for you.
AutoCAD is slow for 3D. It can't deal with many polygons, it gets really laggy and the navigation is quite annoying. You begin working in the XY plane, which is 2D, and once you're working in the Z axis, if you wanna work in, say, the XZ plane, you must specify it, because if you don't, you will keep working in the XY plane, whether you like it or not, because the commands are sensitive of the plane you're working in. Other 3D packages don't give a shit, you go in all directions in one move, but AutoCAD wants you to be specific about this. You can't toggle between views in an easy way, nobody tells you that if you wanna see the fucking sky, to render, you must have your perspective set to 1. Applying materials is a pain in the ass, creating them is equally bad and their quality is questionable. The render options are NOT intuitive. The controls are NOT intuitive. The navigation is NOT intuitive and it fucking sucks. But it has great booleans and I like its UNION command, which creates great topology, but don't even bother. Go 3ds Max or Sketchup, better and less frustrating.