Congrats! This is a huge compliment. I got asked to play alto in the high school jazz band when I was in 7th grade. It was an outstanding experience and I learned a lot. If you are playing bari be sure you have good reeds and plenty of volume. Your job as bari player is to mirror the lead alto player. Often you will have the same exact part as him/her, so you need to match the lead alto's phrasing, articulation, etc. Also, bari parts are often counter to what the rest of the section is playing; you may be playing with the trombones instead of the saxes. Dont let this throw you off... play confidently and you will have a blast!