Pause. Relax. Breathe.
Allow the feeling of gagging to pass then try a bit deeper. It takes practice. You'll find that once you get to a certain depth, the next time you reach it again will be easier, until you are eventually deep throating.

But it isn't something you'll just automatically be able to do. make sure that you maintain control at all times and he isn't forcing it down your throat. If he tries, block with your tongue or hold the base with your hand and pull it back to a comfortable depth.

Also, make his sure his cock, your mouth, and throat are well lubricated... spit is the best lube. The cock should be gliding in your mouth, and very wet. If you dry up due to nerves, try some flavored, edible lube to start.