You need 20" wheels, a lower top tube, small gear ratio, and most importantly,pegs on both axles. Brakes are not needed, but smooth, bald tires are. The handlebars should be narrow and have zero sweep.
Make sure you have a bike with a strong frame, something made of steel, or iron is a good choice, they can stand up to the stress of heavy riding. Also make sure your bike rides smoothly, and that the tires are in good shape so they don't slip out from under you.
One of the best way to set up your bike for a BMX flatland you should get a complete kit which include all of the items that are available for beginners whom never set up a bike before. It is a little more pricey then the individual pieces but it is well worth it.
The wheelbase of bikes become shorter than other types of bikes. This helps to bike spin freely. Main criteria is use of four pegs on the end of wheel axle.
