Go to actual Boxing/Kickboxing/Muay Thai classes. You'll get sparring there.
If ou want to do extra get some gloves, mouthguards and a good friend(prefferably one a lot better at fighting than you, in my case it was a guy who outweighed me by 50 pounds) and spar. A lot.
Thats how I learned to fight. I got the absolute shit beaten out of me most of the time, but I learned how to take a punch and control myself. It's also how I learned that I could choke fools who were a lot stronger than me out. I was doing this when I was about 13.