[dsr_]

Start with an assembly language designed for teaching: MMIX.

http://mmix.cs.hm.edu/index.html

There are books, courses, homework, and lots of free resources -- in particular, emulators.

Then find a project that interests you, and learn what you need to do for that. 68000 assembly? Forth? Graphics on a VGA card emulated in a VM running MS-DOS? All of these things are entirely do-able.

Or find a project that is already in development and contribute to it. The important bit is your own motivation.

[jeffreyrogers]

I don't know if MMIX is that useful to learn honestly. It's RISC like, which is good, but the register saving convention is a bit weird and its not that similar to things that are already exist. Most assembly languages are pretty easy to learn (even x86), they just seem hard because they're so low level.