Download the GNU Development Environment for ARM from hxxp://gnude.sourceforge.net and play with it a bit. It has compilers/assemblers and a debugger with a simulator. You can use these tools to write simple programs and watch them in the simulator; for me this is one of the best ways of learning about a new architecture. Then go to wxx.arm.com and wxx.atmel.com (and others) and watch the application notes and the support files. Atm*l offers a C library for their ARM based microcontrollers which is freely downloadable; take a look at it, it has some interesting thins.