First you need the equipment. A decent spray gun, some paint (try auto paint suppliers, They may have some paint that was mixed wrong cheap or sometimes free), and a compresser.
Once you got that you need something to paint. Old street signs worked good when I was starting out
. A bit more of a legal way to go is to get old sheetmetal. Some of the auto salvage yards have bikes lying around and you can get a tank for a few bucks.
Now you need somewhere to paint. At first you can use your garage. Make sure everything is covered real good and stick a fan in the door and open the windows if there are any.
Probably the most important part is a respirator. Urethanes are a nice durable material to use on a car or bike, but I dont think you want to clearcoat your lungs.
Now practice.