Arduino uses a programming language called Wiring that is a lot like C. If you learn C you'll have no trouble with Wiring. If you want to learn Wiring directly, go here.
Please note that there is no such thing as "learning to program in open source code". Open source is just the decision to make your code public in a formal way. It can apply to any language, and there is no language that is intrinsically open source.
follow the getting started instructions on the arduino website, and load the flashing led program. Play with it to vary the timing a bit. Flashing an led is the microcontroller version of "hello world".