I'd like to give soem tips, if that's the case:
1. Do not just keep spamming Smash attacks. They take a bit to charge up, and leave you vulnerable to faster attacks. Rack up damage with specials and neutrals, and then deliver a coup de grace with a well-placed Smash.
2. Shield often: Can't stress this enough. Some players who do not shield at all are easy pickings for any powerful attack. Shields can change the tide of battle very easily, and they can also be used to roll out of the way.
3. Know what your character does best. Eg: Litle Mac is a fighter who is VERY powerful on the ground, but he has THE worst recovery in the game as well as his weakness is his aerial movement.
4. Spice up your game. Do not simply go on using the same attack over and over again. If you do that, those attacks will lose power, plus you become insanely predictable.
Smash is not a game that you hop into and magically become a world champion at. You need tons of PRACTICE. Most of the tournament-goers have been playing Super Smash Bros. ever since the N64/Gamecube version came out, so they have loads of experience to back them up.