Here’s the basic stuff:
You will need a bike. For beginners any ol’ thang that you don’t mind beating up on a little bit. You can always figure out the details later. Your bike should have one handbrake in good working order, preferably for your non dominant hand.
A helmet, bicycle or otherwise.
Resources to learn about the rules and history of the sport:
Here’s a cool video made by our good friends in Seattle explaining what bike polo is: