Yeah welcome to the boards and welcome to competitive Smash in general. If you're set on having Fox as your main character, I would suggest looking at the Fox guides on the board and watching some videos of high level players. You'll learn a lot that way. Otherwise just practice and get experience playing against good players.
Being a competitive Melee player for years now, I do have the experience and I "know what it takes" to become a competitive Smasher in general, so the transition from Melee to Brawl wasn't that bad. But if you haven't played Melee competitively either, don't worry about that. Brawl is a new game, with new things to learn, and if you're starting now, you won't find yourself too far behind the rest of the competition.
Don't let anything get you down. I remember when I was trying to learn some of the techs in Melee (Wavedashing, waveshining, etc etc), it took a while and even once I "learned" them, I didn't really see a huge improvement in my game, which discouraged me a bit. I just kept practicing until I was able to utilize everything effectively, and kept playing other people and entered tourneys to gain experience. It took a lot of work, but if I had stopped right when I first became discouraged, I would have never been even half as good at Melee as I am now. So- just keep plugging away, and don't let failures get you down.
And just remember to have fun with whatever it is you decide to do. That's the most important.