A lot of people would probably suggest you use Solidworks (because of how common it is in industry), or Autodesk Inventor (because of its simplicity/educational resources). I’m a fan of Creo though, so hopefully I can give you some starter tips.
PTC’s FIRST Community is probably the place to start for you: http://communities.ptc.com/community/academic-program/first
They have videos and documents explaining Creo for students and teams who are new to both their software and general product design principles. The videos can seem a bit dense, but you can always pause them-- don’t be afraid to back up.
After you have the basics down, in my personal opinion the best way to learn a new CAD package is to come up with a project or concept you want to explore, and then explore that design-- there are videos and tutorials out there for almost every feature in any of the FRC “big three” packages-- side note, many of the tutorials from Pro/Engineer still apply to Creo once you translate the UI-- some of the tutorials are good, some are weird, some are just plain bad, but you’ll learn a lot about the software this way.
If you need any advice, feel free to PM me or email me (after you’ve PMed me). If I can help you, I’ll try my best to.
So, to summarize, the tutorials on PTC’s FIRST Community site are a good place to start, then try PTC University (which has good content organized terribly), and then try Google/YouTube for tutorials. After you have the basics, pick a project (ie, creating a CAD of your 2013/14 robot), and use tutorials to figure out how to complete that. Bon chance!