While there are many ways to give head, I feel like there are few greater pleasures than the reactions of a super hung guy when throating him for the first time.
Here are some things you can do to help that along:
1 - Practice. Get yourself a toy that's a little thinner than him and work a few times per day on learning to throat it. Gradually work it deeper until you start to feel the gag reflex just begin, then back off before your gut tries to heave. Use the points below to help with that.
2 - Align properly. If he's got a bend in his shaft, or a head that raises or droops down, position yourself so that his contours match the bend of your throat. On your knees in front is great if he has a downward bend. If upward, then getting in "69" position (or facing that way but with your pelvis off to one side if he doesn't suck you) works better. Once you're more experienced and you're ready to let him take over, laying on your back for a proper skull fucking may work well, but hold off on that until you're much more confident, or into a dom/sub scene.
3 - Use your tongue. Your tongue is your greatest asset when giving deep head, or any head for that matter. Pushing it out as far as you can will ease the bend in your throat, then you can pretend you're "grabbing" him with the tip of your tongue and pulling him in. Sometimes if he's really big it'll take three or four of those "grab & pull" moves to get him all the way down. The "swallowing" mentioned by find83 is really hard at first but I've been able to throat some truly enormous men like that.
4 - Be prepared. Be well hydrated so you can produce plenty of saliva to lubricate him and your throat. However, it's best not to eat or drink for a few hours beforehand so that if your body heaves, you won't risk making a mess.
5 - Be confident & relaxed. This is something that you're ready for, and he's going to love it. Know that you can please him and don't tense up. Know beforehand that once your body is accustomed to him, throating will become so much easier. If you do gag (and don't make a mess), that's just a little discomfort that you can learn to push through, like the good pain you feel when exerting many other physical efforts. Some guys really love the feeling of you gagging on their meat, as long as you don't pull off and quit!
Some guys resort to chemicals to relax further (poppers, alcohol, molly, etc) but I'm not a fan of potentially diminishing the experience and putting myself at greater risk.
This is so much fun, and so worth the effort to get good at, but not all headmasters can throat all men. There are limits to our anatomies, but many suckers never get close to finding those limits.