If you are playing with a singer, the singer will sing the melody. You can play subtle fills behind them while they sing the melody, but don’t get in the way!
NEVER read music from a phone. Try to memorize the song you’re playing!
If you are the only horn player, you will play the melody and likely take the first solo.
If you are with a group of horn players, you may: play the melody in unison with the other horn players (best for bebop melodies or tunes written by jazz musicians), play the melody on your own for a portion of the song such as the A section and let another player play another portion such as the bridge (best for Tin Pan alley songs and ballads). Harmonization and background parts are also great if another player is playing the melody and the parts you play make sense.
On tunes at a medium or faster tempo, expect everyone (or almost everyone) in the band to take a solo starting with the horns, followed by piano, guitar, bass, and drums. If you take the first solo, don’t take a million choruses but also don’t just take one chorus. Solos that follow will generally match or approximate the number of choruses you’ve taken.
When others are soloing, stand to the side of the stage.
Follow the form and listen closely to everyone as they’re soloing. Never noodle on your instrument as others are soloing. Be ready to come back with the melody! Make eye contact as solos near their end to get a sense if they soloist will continue or wrap up. The universal sign to bring the melody back is tapping your head.