First thing you have to do is wait for the Meet The Contestants mission which will occur quite early in the game. After killing Leon you will gain access to his trailer which is at the NorthWest end of the Silver Strip near the entrance to the Yucatan Casino. Inside is a motorbike which you can customise with certain weapons and paint. Simply bring a can of spray paint (there are a couple on the small rotunda with the other bike on the Platinum Strip near the entrance to the Arena) to the trailer and combine it with the bike.