U - L - T - R - A r e a l i t y . info
If you're new to programming or the C programming language, click here. This link will take you to a basic C programming tutorial course, and by the end of it you'll have simple commandline programs compiled, have learned the fundamentals of programming concepts, and some helpful mathematics that will come up later.
If you're already up to speed and comfortable with the C programming language, click here. You will be introduced to the hardware of the Ultra 64, how it differs from that of a PC, and how to best think about the system's hardware when programming for it.