A lot of the FRC related things are
here at Andy Mark
try finding Tutorials online to get used to the software. the more you design the better you will get at using it. I personally like SolidWorks, I taught myself how to use it and everyone that I have referred it to Have LOVED it. I am not sure if you can get it through a download, but I got it from filling out an application from SolidWorks and they sent it to me in the mail, completely free.
Look at the other CAD models of robot, chassis, and more to get an idea of how people design robots, and then you should start to design something, most people do a chassis their first time.
Here is a link to this years KOP and competiton field.
If you have more questions you can PM, and I can help you some more
Good luck!