First of all, the person who said you were terrible and should stop playing Smash is a pretty big jerk.
Anyway, before you start playing in For Glory, make sure to learn your characters bread and butters first. Checking the Duck Hunt Duo
category here on the forums should help you out some, and you can also ask questions to other Duck Hunt Duo players who were once in the same situation as you.
Here's a somewhat-basic guide I found in that category, which should give you somewhat good idea on what you should and shouldn't do when playing as Duck Hunt. (In any case you don't play Duck Hunt, just replace Duck Hunt with whichever character you prefer, I'm just basing this off of your profile.)
Most importantly, practice makes progress. Learning your favorite combos may be one thing, but aren't going to mean much if you can't preform them in battle. Once you feel confident, maybe jump into some less-serious online modes (For Fun, Free-For-All For Glory), and practice against some other players.
Also, don't be afraid to lose. Even the best of players crack under pressure sometimes. If you start getting angry at For Glory, don't be afraid to take a break from it. Play For Fun, Smash Run, whatever calms you down. You should also have fun while doing this. Smash Bros is a video game, and the main goal of that is to keep you entertained, not to upset or anger you.
Sorry if this sounds really dumb and cliched, but from previous experiences, this is what bumped my Win Ratio from a 50% to a 70% (and still rising). Good luck and have fun!