How do I compile a .c file on my Mac?
Ondrasej is the "most right" here, IMO.
There are also gui-er ways to do it, without resorting to Xcode. I like TryC.
Mac OS X includes Developer Tools, a developing environment for making Macintosh applications. However, if someone wants to study programming using C, Xcode is too big and too complicated for beginners, to write a small sample program. TryC is very suitable for beginners.

You don't need to launch a huge Xcode application, or type unfamiliar commands in Terminal. Using TryC, you can write, compile and run a C, C++ and Ruby program just like TextEdit. It's only available to compile one source code file but it's enough for trying sample programs.