You'll need:
Bass
Strap
Instrument Cable
Tuner
Amplifier (can either be a combo amp with a power section and speaker in the same box or a 'head' and 'cabinet'.)
- For small to medium gigs, somewhere around 300-500 watts into a 4x10 or 2x12 should be plenty.
- For a head and cabinet setup, you'll need a speaker cable that connects the two. What type depends on the connections on the head and cabinet. 1/4", Speakon, and Banana connectors are common.
You'll also want to bring your own power strip or extension cord.
If your amplifier has a direct out (a 3-pin XLR jack on the back), then the sound guy can use that and you'll have some tonal control over the house sound. If not, the sound guy can provide a Direct Box, which converts your bass signal to an XLR jack and you go directly into the sound board, bypassing your amps tone. Or you can have the sound guy put a microphone on your cabinet.
Other things you should bring, but aren't necessary:
Backup bass
Extra strings - with a string winder and wire cutter
Extra instrument and speaker cables
Extra batteries or adaptor for your tuner
Duct Tape
Sharpie
Paper (for setlists and groupie's phone numbers)
Case for your basses
Extra extension cords or power strips for your guitarists
Your own nice microphone, stand, and cable...or a quality Direct Box that is better than what the sound guy provides
a bass stand, so it's not just on the floor or leaning against your amp.
Hope that helps.