How do i improvise in jazz band?

Start by playing the 3rds and 7ths over all the chords in the song. For example: if the progression is cmin7/F7/BbMaj7, you would play Eb and Bb over Cmin7, A and Eb over F7, and D and A over BbMaj7. These are not the hippest sounding notes, but they are always safe to play. After you do this and understand the movement of the changes, and the 3rds and 7ths, improvise and add notes from the diatonic harmony (ie from the scale, in my chord progression, it would be Bb Major), and try to resolve to the 3rds or 7ths of the chord. After start experimenting with Bebop scales, which are, with root C: Bebop Major: C D E F G G#(Ab) A B Bebop Minor: C D Eb F G A Bb B Bebop Dominant: C D E F G A Bb B Also a great idea is to listen to a lot of jazz, and figure out what they are doing (transcribe)!!!

you need to learn chord scales every chord has a scale to go with that chord Bbmaj7 chord scale= Bb C D E F G A Cmi7 chord scale= C D Eb F G Bb F7 chord scale= F Gb G Ab A B C Db D Eb

yea if the song is in the key of b flat then you can play every note in the b flat scale and it will come out ok.i would start with simple jazz rhythms and arpeggios.you might also want to do some research on chord progressions to adjust to changing keys instantaneously.