What kind of bike polo- grass or hardcourt (tarmac)? If it's grass theres loads of rules about priority, passing etc, the hardcourt version is a bit simpler:
* Can't put your foot down (or you have to tap a "penalty spot" on the edge of the middle of the court)
* Only score by hitting the ball with the small end of the mallet
* Don't be a dick (riding at full speed into someone is generally a bad idea)
Though if you've never played before there's nothing to stop you playing hardcourt rules on grass so it hurts less when you fall off. Any bike will do, doesn't have to be fixed, goals can be traffic cones or whatever comes to hand.