well, there are TONS of resources on the net where you can learn C, type in C programming tutorial in any major search engine and you are going to get a TON of hints. Supposedly the one on the way stuff works website is pretty good. If you have the time before you need to be writing a ton of your own code, though, I would recommend getting a book on it at your local bookstore, or amazon.com or something like that. Also, there are many other FIRSTers out there that are more than willing to help teach you........such as, for example.......
Me 
I'd tell you the books that I learned from, but they are currently loaned out to teammembers to help them out.
Once you know the basic commands and syntax etc, the best thing you can be doing is reading through the default code and figuring out what each line, function, and algorithm does. Then try modifying it to do something a little different. Test, debug, and do the same for another function. (obviously some functions are more easy to do this with. I wouldnt go messing around with the initialization functions and such as of yet). Then, start trying to write your own functions. Before you know it, you'll be able to program in C!
Any questions, comments, etc feel free to email me, PM me, IM me, etc etc. If for some reason there is trouble looking up my profile or if you feel lazy my email is
RobotCoder@yahoo.com
Good luck and have fun!
-Neal Orman, Lead programmer for team 1002: Circuitrunners