c (and its variants c+, c++, etc) is one of the most widely used languages for application development under the unix operating system. As such, it is almost certain that a c compiler is already installed if you are working under linux or mac os. The commands "whereis cc" or "whereis gcc" or "locate gcc" or spotlight/gcc or spotlight/cc (on mac) should point you to the compiler. If by some odd chance you are sure that a c compiler is not istalled, you can download and install gcc from gnu.org. See the wiki in this case. You can also most likely find a precompiled package for your unix operating system. Under linux, "yum install gcc" should do it. There should be analgous "disk image" packages available for mac.
C TUTORIAL
A good c language tutorial can be found here.