Start with a "Learning to program in C" book for computers not PIC's.
>From there, write a couple of simple C programs (usually examples in the
book).

Once you've cracked the variable types and how to write functions using
simple i/o (eg keyboard input and screen output) then move over to PIC's.

There a dozens of free C compilers available from the web. (Keep to the
simple ones).

Once you've got the text "Hello World" up on your screen, and written a few
functions to compute fahrenheit / celcius conversions then you'll be well on
your way.

Remember... basics first. Don't try to run before you can walk. Mistakes are
easy to make with C when you start out. Eg, some code ends with a semi-colon,
other bits of code don't need the semi-colon. Certain ascii characters have
special meanings.
Keep it simple, learn the philosophy behind C.

It's a fantastic / powerful language, you're gonna love it.

Good luck.

Darren