There's a lot of steps to getting over tourney nerves, and no two people get over them quite the same way. However, there's some things you might want to focus on...
Firstly, the more you go to tournaments, the more comfortable you'll be. Try and go to as many as you can.
Secondly, don't expect to play perfectly in bracket, or even in friendlies. You almost never do. Keep this in mind when you're practicing, too -- if you can L-cancel consistently 95% of the time when you're practicing at home, you can probably L-cancel 75% of the time in bracket, 95% when you're playing amazing.
Thirdly, don't go to a tournament with the goal of winning the tourney, or a set, or even a game -- not for now, at least. Go to have fun, meet people, make your game better, whatever works for you. Then, when you're starting to get good enough to place, you can start focusing more on winning.
Fourth, don't get caught up in individual mistakes. You start playing a match with a real person next to you, you hardly know them, and OH CRAP YOU JUST MESSED UP YOUR LEDGEDASH AND DIED (or whatever the case may be). Don't think about it. You messed up, and there'll be time to grind away the mistake later. Just for that moment, though, save it somewhere in your brain and focus on playing. Then you can write down what you need to work on, or go home and start grinding, or whatever.
Finally, start focusing more on the game, instead of the people around you, or the noise, or that food waiting at home, or that ONE option that you CAN'T seem to hit because your opponent WON'T LET YOU! (eg peach's downsmash) Nerves come with distractions and worrying. If you're focused, you won't get them half as easily.
I hope this helps, even if just a little bit
~EZLO
