Before you start playing be familiar with the hierarchy of good band manners.
First, be in tune. Everybody’s uncomfortable when something sounds sour and is out of tune.
Second, play in rhythm. Groove with the bass. When in doubt, pause and listen for the next down beat. Play as little as possible. I’m not sure of the author of this quote but it’s made a lot of sense through the years “It’s not what you play, it’s what you don’t play”. The less you play, the easier it is to play on the beat.
Third, play the right chords. Do your homework and be prepared. Learn the chord chart before you come to rehearsal.
Fourth, learn the notes of the melody- they’ll come in handy at a bunch of different times and on different levels.
After you start playing follow these rules when possible:
First- Start together and end together. Beginnings and endings frame the song you present on stage. It’s the first and last thing that the audience hears and remembers. If these are tight, other “discrepencies” will be forgiven.
Next, when someone starts singing, play softer so that the singing is featured. Create a sonic support/safety net for them. The vocalist is is the most exposed member of the band- make them sound good. In general, build volume in the chorus.
Lastly songs are cycles of repeating musical ideas. Try to add something different each time you play through a verse or chorus or bridge. Something simple- an extra note, a different voicing, a rhythm motif. Add interest to the performance.
Be kind to your band members. Support them musically. Make sure everyone gets a chance to shine. Playing music can be as challenging as it is rewarding. We are at our most vulnerable when on stage. Make it fun for your bandmates and they just might return the favor.