Watch 1678's Fall workshop videos
http://www.citruscircuits.org/fall-workshops.html
Watch 973 RAMP videos.
After that, start with a drive train. decide on what type of drive you'd like (*cough*cough* 6 wheel tank). After deciding on six wheel tank drive, find another teams CAD from previous years. I'd Recommend 973 robots from previous years. Don't be afraid to borrow ideas from other teams in earlier years. As you likely know, FRC tends to repeat game pieces every few years (2012 and 2016).
This has saved my literal countless hours, before moving to CAD, start on paper. Sketching ideas on paper before moving on to a model will almost certainly help iron out a lot of kinks early on. That, and drafting is a timeless skill every designer should master eventually.