Autodesk’s 123D Design is a great entry-level CAD program. It has most of the tools you need to create your 3D models, while still being easy to use. The other big benefit of using 123D Design is that it is well supported and documented. This means that no matter what you are trying to do you can find some tutorials or guides to help you out.
If you are a beginner Autodesk has created a set of quick start tutorials that run you through the basic functions of the program. These tutorials teach you what you need to know to get started on your project quickly. You can find these quick start tutorials here.
If you have already used 123D design before and maybe it has been a while or you just can’t quite remember how some of the tools work, there are tutorials for that too. Adafruit has done a short video tutorial on several of the frequently used tools. This video doesn’t go over the details of how each tool works but rather quickly shows them in use. This lets you see what tool can be used where and then if you need additional information on that tool it is very easy to google. The video can be found here, the version of the software is slightly older but the methods are still very similar.