First off, read this thread:
It will give you some idea of playing smash competitively (If you are taking it seriously that is. If not, it's still worth a look)
Secondly, Since you are playing Roy, I will warn you that you will be playing at a large disadvantage Roy is considered one of the worst characters in Melee and you may just plateau. BUT DON'T LET THAT SCARE YOU AWAY FROM ROY! As long as you are having fun playing Roy in Melee, you are set.
Thirdly, don't expect to do well at your first tournament or pull a Ken (Winning your first tournament). In fact, I think the first goal for any new smash player would be to try to make it out of pools.
Practice, Practice, PRACTICE PLS!!! Even if you think you have an advanced technique or a combo down., practice it. Practice a technique until it's second nature, and you don't think about it. Also, make sure you learn how to apply the advanced techniques like wavedashing, L-Canceling, Dash Dancing. If you ever need help with Roy, feel free to browse his subforum on Smashboards.
Finally, have fun! You are playing this game competitively because it's fun. Go to tournaments and interact with other people. Make new friends and stuff. If you don't find it fun, find a way to fix it or call it a day.
I wish you good luck on your Journey in Melee
Welcome to the good life (as Squire stated).