Let me start with what sparring isn't. It is not a regular boxing match. It is not designed to show who is the queen of the ring. Nor it is for to prove who is toughter among two opponents. Sparring is practice fighting.
The scene is set up by the coach. He or she picks two people to face each other in the ring. The coach tries to match people up by weight and height. Sometimes, experience will figure into it as well. However, a fighter will less experience will learn more if paired up with someone who has more experience, even if it is just a little more. The idea is to get better at punches, footwork, movement and ring generalship (how a fighter controls the action inside the ring). When the bell rings, the coach stands either outside the ropes, or inside the ring. They give instructions to the fighters as they move around.
Most are nervous when sparring for the first time. This is normal. Relax. Remember, it's not meant to be a slugfest. The coach wants the fighters to work together and learn, but they will call a halt to the action if things get out of hand. However, people do get hurt from time to time. Bruises, bloodied noses, and busted lips happen. Once in awhile, a fighter gets knocked out. It happened to me once. I had my hands down at the wrong time, and a right hook connected with my jaw. If a knockout does takes place, that sparring session is ended.
Fighters also have the right to request that the other person slow down the sparring if things get heated. Don't be afraid to say something like, "Look, I just want to move around and work on this or work on that. Let's go light." If the other person wants to have a ring war, it's also okay to bow out and say, "Uh, I'd rather spar with someone else." Most times, if the coach sees something like this, they will admonish the fighter who wants to play Christy Martin for not working properly with their opponent. They may also stop the sparring session.
When fighters spar regularly with the same people in the gym, they'll learn other people's moves. Unfortunately, sometimes that leads to fighters becoming too comfortable and predictable. That situation can be detrimental, especially if a fighter is training for an amateur bout against a fighter they don't know. Some coaches will invite fighters from other gyms to spar with their fighters, or they will arrange to take their fighters to other gyms to get in sparring time there. It's good to shake up the routine and have fighters get used to other people's styles in the ring.
After the sparring session, the coach will make their comments about they observed in the ring. They will make suggestions to both fighters for improvement.