Big props to you for asking this sort of question!
A song is ready when the band can play it right the first time, no excuses! There are no "do-overs" on a gig.
Choose the set lists in advance and stick to it, at least for now. The band is ready for a gig when it can play the sets start to finish with minimal time in between each song, too keep the party going. This will prepare the group for common mishaps.
Broken guitar/bass strings -- Each player should have a backup guitar or bass, tuned and ready to go at all times. Strings should be replaced at set break.
Tuning -- Guitarists and bassist should be able to make a tuning adjustment in 10 seconds or less. In-line stomp box tuners with a bypass feature are a professional necessity, not a luxury. No crowd wants to hear your band tuning, or wait for it to happen. Ever.
*If someone needs to tune, he/she should let you know mid-song, and then the tuning can happen in between songs. Many times, tuning can happen mid-song, in moments when there aren't guitar or bass parts playing.
Tempos -- As the drummer, it will fall on you most of the time to count off the next tune, and nail the tempo. Use a metronome, so that adrenaline, nerves, and the tempo of the preceding song do not influence your judgement. Even in songs that do not begin with drums, be prepared to count off, for the sake of consistency. Only take this advice if you wish to avoid lots of arguments regarding tempo! ;)
On the gig -- No rehearsing once it's gig time! Many players will play their part immediately prior to the band playing song, which is amateurish. Don't play the song until it's time play the song!
If a fight breaks out, stop. The bar will know that something is wrong, and hopefully do something about it. When the situation has been handled, don't repeat the song, move on to the next one.