If you've never programmed anything before but can follow simple directions you can still learn to code for Quake. If you have programmed in C or C++ or similar languages it'll be a snap to pick up QC.

Coding newbie or seasoned pro just new to QC you should check the document on this page
http://www.gue-tech.org/quake/qcrm/index.html

To actually start coding you'll need the sourcecode and a QC compiler. There are many versions of each available. For the beginner the basic ID 1.06 source should be used since its compatible with all the available tutorials. The compiler I recommend is FrikQCC available from here If your an advanced coder you'll want to check the QC optimizations section to make your source a bit smoother and efficient.

Here's a few links for the newbie coders...

Getting Started with Inside3D
[ The basics for the newbie coder ]

Inside3D's QC tutorials
[ Good collection of tutorials ]

QC Extreme
[ Even though its no longer updated it is still an excellent source for quakeC info. While your there check out his mods and maps. ]

AI Cafe
[ Ever felt like making your own bot? This is a good place to start. Coffee has his simple "tutorbot" there and tutorials to show you how to improve it to meet your needs ]