Getting very good a Brawl is pretty tough actually. I've been playing the SSB series for 7 years and there are still people that are much better than me.
What I usually do is a 3 stock match, no items, on Battlefield. I play a random character against a random level 9 computer. This helps me learn all of the characters (moves, weaknesses, etc). From that, I picked my best characters and practiced with them.
It helps to look online for basic combo tips and tricks for your character and practice them in training mode. Practice spiking (if your character can do so), edge guarding, and anything else that seems useful.
The one secret to Brawl is to time your attacks. The best Brawl players have the ability to predetermine their opponent's moves. You have to know what the other player will do BEFORE THEY DO.
An example trick of ^. Usually after you die and jump down to your opponent from your spawn point, your opponent will dodgeroll behind you. If you already know that they will do so, hold off on that Fsmash that you want to do so badly. Wait one more second for your opponent to roll behind you and THEN hit them.
Brawl is all a mind game. Unlike Melee (where you forced your opponent to make mistakes), you have to wait for your opponent to make a mistake.
Sig and Avy made my me!!!Brawl FC is 3394-4531-4018