Unlike PBASIC, C is a very standard and widely used language. You can go to your local bookstore and probably find about a dozen different books on C. Don't get a C++ book. Years ago I used Ivor Horton's Beginning C++, it wasn't the most readable book out there but it was very comprehensive. I imagine that there is a C version available. There are also numerous online tutorials available, some good, some awful, just search google.
If you're looking for a compiler to learn C, I recommend Codewarrior Learning Edition. Its for Mac/PC and its very powerful and easy to use and comes with a lot of different reference stuff. Also check out
www.codewarrioru.com, they have free programming classes online with a real instructor, forums to ask questions, etc.
You could also look into taking a basic computer science class at a local high school or college, many of them have switched over to Java, but there are plenty of holdouts.
Honestly, if you want to learn programming or computer science, PBASIC is a terrible language to start with. If you want to do anything in CS beyond FIRST I'd highly recommend taking an introductory class taught in Java. Once you know Java and have a good background in CS writing code for FIRST is fairly easy to pick up.