First you need to pick a CAD program; they are mostly completely different from each other. Learning to use one doesn't translate into skill in using a different one.

Do you want to do work in a commercial field? Than you should go with an industry standard: solidworks or autocad inventor for solid modeling, autocad for 2D work. Down side: $5000+ a copy. You can get student versions if you are a student, that will last for a year. Might be worth taking a course at the local vo-tech college to get you access to studentsiftware. If you are a veteran, solidworks will also let you get the student version.

Do you want to design stuff for yourself? Now the field opens up, and the options are much less expensive. Rhinocad and alibre are two common programs.

How to learn? I went through solidworks, and I started by working through a bunch of Paul Tran's and then David Plancard's books, which are excellent. These programs are not something that you will learn by just fiddling with them. You need a structured approach.

SolidWorks 2014 Part I - Basic Tools: Paul Tran: 9781585038534: Amazon.com: Books

I started learning rhino3d, than went to Alibre, than to autocad, finally to Soldworks. I like Solidworks the best; i just seems to make sense to me and is relatively easy to use.