Hey Night! Welcome to Smashboards!
Tourney nerves are normal, so don't stress too much about them at first. BUT, there are many many ways to get over them.
First, play to learn, not to win. You've probably heard this somewhere else, but if your primary goal is to learn, you won't get scared of losing -- as long as you've learned something. Of course, try to win, but that should not be your primary goal. If your opponent is doing something you can't get by, take a mental note of it so you can figure out how to counter it later. This also really helps with improvement, so there's no reason not to.
Come in with a plan for how to beat each character. If you have that, then it will be easier to focus in on the game.
Practice against real people -- friends, your grandma, your 2 year old sister, whoever -- as if you're in a tournament. Play a best out of three set, and don't just select random to choose stages -- actually ban stages like you normally would. At the very least, this will make a tournament seem more familiar, and most likely more comfortable.
When you play a match in tournament, treat it like a friendly. If you think you won't play as well because you're thinking of it as a casual match, refer to what I said above. You should think of it like a match where you're relaxed because, well, you'll be more relaxed. If you mess up, you can shrug it off and keep going (that being said, you should also take note of that so you practice whatever it is you messed up).
Most importantly, just go to more tournaments. For a while, you'll get tourney nerves, but eventually they'll begin to feel normal. I can't stress this enough -- the more you play in tournament, the less a tournament will make you nervous.
I hope that helps!
