Here are some thoughts about entering this field. These are guidelines and not definitive.
Practice, practice, practice! Being the best on your instrument is really important. You might be one of the best here, but it is a whole different environment when you get to the collegiate level. You will be competing with others who were the best at their schools, too!
Embrace fundamentals! Play long tones, lip slurs, tonguing exercises, overtones, rudiments (percussionists), scales, etc. and practice them daily. It will help get you into festivals...and the college that you want to attend!
Take private lessons on your primary instrument.
Audition for Honor Bands. While we have a wonderful high school music program, it is integral to your success to perform in as many higher level ensembles as possible.
Perform, perform, perform. Play in as many ensembles as you can. You never know what experiences you may need.
Learn to play the piano. The earlier you learn to play piano, the easier it is.
Sing, sing, sing! Sing in chorus, sight sing, take vocal lessons, sing solfege. You will use it in college and beyond.
Take music theory classes. You need a strong background in written and aural theory.
Learn an alternate instrument. Play it in another ensemble like marching band, jazz band or a small ensemble.
Learn music technology. Learn as many music programs as you can. Music technology is the wave of the future.
Listen to professionals perform. Listen to recordings, go to performances. Take master classes at local colleges if they offer them.
Record yourself and then listen to yourself. It will sound different! Have teachers and non-musicians critique your playing. Don't be afraid to hear criticism...embrace it because it will make you a better musician.
Learn the standard repertoire for your instrument. You will be able to use it for your auditions.
Get a step-up (intermediate level) instrument. If you can't afford a step-up instrument, at least get a better mouthpiece. It will make you sound better. Once you get to college, your teacher will guide you toward the professional model instrument that is right for you.
Do practice auditions with as many people as you can. It will help you conquer audition jitters.
Help with the middle school ensembles. It is amazing how much you learn when you are on the other side of the instrument!
When getting ready for your college audition, be prepared to play at least two pieces (sometimes three) of contrasting styles, sight read a piece, and play scales. Percussionists should be ready to demonstrate skill on timpani, mallet instruments, rudimental percussion and auxiliary percussion instruments. Be ready to sight sing and play piano as well. You may also be tested on your music theory background. You may also be asked what your career goals are to see if you fit into the program's philosophy. Some colleges recruit students into their program in small ensembles like brass quintets, woodwind quintets, etc. so experience in those types of ensembles is very important.
You may be thinking of going into music because you had the time of your life in marching band or jazz band. You made life long memories going to festivals and performances. But when you major in music it is a full time job that extends way beyond playing in band. If you don't have the desire to practice for hours a day, wake up for 8am theory classes, do extensive research, work on piano, and singing while all your dorm mates are hanging out, then this is probably not for you.
Most students do not realize this until they go to college and wind up wasting at least a semester, if not longer. Be very sure that you are willing to do all of these things!
Here is a link with some observations from a student and his mother about selecting a college. http://sites.google.com/site/newalbanyhighschoolband/college-admissions This offers some great insights into what you should be doing when choosing a school.