First things first, do your research. Look into what a Dominant does, check out the different kinds of kinks you’d be into and prepare as much as possible. If you want to do impact play, practise on a cushion first to get your aim in. Attend workshops at your local dungeons and BDSM clubs and learn all you can.
When you feel you’ve done enough research start looking for a Sub. You can check profiles of people close to you on fetish.com or post a personal ad to find someone into the same thing as you. Be totally honest with anyone you chat to, let them know you’re new. After all honest communication is the key to a good BDSM relationship, well, any relationship in fact!