I'll agree with GeneralWoodman. Though I'd say you should start off in Free Play chat, which is one of the chat rooms on AllisBrawl's website where you can find matches for yourself. Play there first, before the actual ladder, because that way you'll be able to learn more about this game's mechanics and your objectives in a match, without being ranked for it.
Jigglypuff isn't a "bad" character. But to play her at the level in which you can compete against people who main higher tier characters(such as Meta Knight, Snake, Diddy, et cetera), you'll need to learn and develop an understanding for the game's mechanics, much moreso than the people you play against.
With this in mind, I'd like to say that you shouldn't immediately begin playing Brawl competitively using Jigglypuff, but choose a character in which enables you to jump right into the feel for this game. I'm going to go ahead and refer you to either R.O.B. or Diddy, because they have a much higher number of options than Jigglypuff does to execute in a match. It'll provide you with a pretty favorable learning curve, due to the several requirements needed to play them effectively; the ability to appreciate and realize the way different characters get more options as far as their movements are concerned, getting to learn the projectile-related aspects of Brawl, et cetera.
If you want to play Jigglypuff on the side, that would also be cool, but beginning your competitive career maining a character which requires an understanding and knowledge about the game that would take an average of about a year and a half of attending tournaments to exercise effectively will have the least favorable of impacts on your outlook, attitude, and progress.