Personally, I think you should dive right in to making a game!
Think of a good idea, then, research (or ask the forum for opinions) on how difficult it would be to make that sort of game, and then just...go fot it! When you run into problems, take a peek at the manual, and if that fails, ask here again, or keep trying to figure it out. Never give up!
Ofcourse, thinking "I WANNA MAKE A MMORPG WITH REALLY COMPLICATED SYSTEMS AND WOAOWHOH" is gonna be difficult to dive into, but...well...do what you want, really! You're only limited by your own imagination; there's always a way to create a 2D game in C2!