Here's a great collection of resources that'll help you get better at all things band. You'd be amazed how much you can learn just by using the sites a few minutes each day!
Teoria - Great collection of music theory lessons and resources. Music theory helps you learn the science behind the music; why do certain notes work together? How many beats are there in this measure? How long do I hold an 8th note for? How do I count this rhythm?
Rhythm Trainer - Fun, easy, free, and quick way to help you learn your rhythms! For real... try it today!
Music Theory - Another great free website full of trainers and games about music theory.
Musicards - Yet another fast, free, easy way to practice tons of different music exercises!