My advice: Start going to some smash-fests first. Tournaments are a very good way to learn, but, what I did was: get to know people first. Get into the scene. Know which good players are in your neighbourhood and play against them. Not only do you learn who your opponents are, but they also get to know you, so, you don't have to deal with being completely unknown at a tournament. It's nice to have some people you can talk to to blow off some steam.
That aside, if you do want to go to a tourney, do not play to win but to experience. Improvement will come along with it. It helps if you just enjoy playing and just play to get a feel of tourneys, instead of taking some stocks off.
Part of the thing with tourneys is the pressure and not really wanting to win (well, you know, you still want to win, but, not the main focus) helps.
Besides that... well, prepare some strategies. Get to know your chars (like been said). Know which chars are hard for you to face and practise against those. Etc. Good luck ^^