Just check out general AVR sites. (www.AVRfreaks.net is the best I've run across) Read the stickies there FIRST before asking questions. Their projects section has TON of C code to read (so do the Atmel appnotes) which can help you get a grasp on things. But if you don't know ANY C at all definitely study ANSI C a little first.
**broken link removed**
Is a freeware 32bit Windows based C compiler very closely based on ANSI C which is a perfect learning language (it's free and there are a lot of examples) however that is for the PC only.