Setting a big goal (perhaps bigger than you think) and tackling it by learning "anything and everything" without a proper strategy probably isn't the best way to do it. I recommend starting off a little "smaller", for example replacing a character's costume in Project M 3.6. (A place that a lot of people start, I think.) There's bound to be some guides for that. Head over to BrawlVault where you can find a bunch of character costumes. Once you do replace costumes, you'll probably be wanting to replace their pictures that appear in-game. You can learn that
here.
Not too complicated stuff. This'll help familiarize yourself with the parts that make up a custom Project M build, as well as knowing what files a character uses.