I started out learning MIPS which is a very compact 32-bit architecture. It is a reduced instruction set, but that's what makes easy to grasp for beginners. You will still be able to understand how assembly works without getting overwhelmed with complexity. You can even download a nice little IDE, which will allow you to compile your MIPS code: clicky
Once you get the hang of it, i think it would be much easier to move on to more complex architectures. At least that's what i thought :) At this point you will have the essential knowledge of memory allocation and management, logic flow, debugging, testing and etc.
