My knowledge is about to make me look like a whole other kind of “professional” here, but in the name of education:
If you’re starting off taking him in your mouth like you’re trying to inhale him, you’re gonna have a tough time. Shots like that are best left for porn and the true professionals who work together and know exactly what they’re doing, often without any real pleasure in mind.
Obviously every man is different, but my husband loves it when I start with long, slow licks along the bottom of his shaft. Make plenty of eye contact while you do, but confidence, not crazy: you want him to feel like you know what you’re doing, not like you’re about to bite his dick off.
Plenty of kisses too, and small sucks on the sides and tip of his penis, but again, not taking him all the way into your mouth. You want to build up pleasure and sensation for him.
If you salivate, don’t be in a rush to swallow it: my husband absolutely loves it if I collect it on my tongue and “spit” it (more drip it, not a full-throttle “hawk tuah” event) onto his penis. It adds to lubrication and a kind of carnalness of what you’re doing. Lubes are great, but in my personal opinion, a clean penis tastes even greater.
Take him deeper into your mouth only if/when you feel more confident, and for all that is holy, go slow. Remember what I said earlier about not trying to bite his dick off? That still applies. Open your mouth wider than his member and envelope it around him, and cover your teeth with your (wet) lips. Don’t be afraid to lick your lips to lubricate them some more, but make sure they act as a safeguard between his penis and your teeth.
Speaking of, a dirty little secret will leave many a man wondering where you learned your skills (but I’m gonna share with you anyway): underneath his glans, there’s a band of skin called the frenulum (it’s more prominent when he’s erect). Run your tongue slowly from bottom to top, in fact, you can even go lower and stimulate his urethra at the same time, which is equally sensitive and likely to drive him insane. Use your tongue only - this is a precision-guided mission that’ll make your man’s eyes roll to the back of his head.
If you’re okay with the taste/texture of cum, you can suck his glans (the head of his penis) while he ejaculates. Again be gentle though, and mind your teeth.
Don’t forget his balls: those little orbs love some attention too. Kiss them, suckle them, lick them… nibble them if he’s into that.
You can use your hands: no-hands blowjobs look great in porn, but I’m sure he’d rather you use your hands to give him the sensation of a deeper blowjob than risk a collision with your teeth. If not, then, good luck buddy.
Finally, position matters: my credibility as anything other than a professional cocksucker is already on the line here, so I might as well go whole hog. Personally, I’ve never found it particularly easy to give a blowjob with him led on his back with his willy in the wind (or in my mouth, for that), and it’s never been easy to just bend at the hips either. The best position for me - and much to my husband’s delight - is on my knees. Experiment, and find what works for you. The more comfortable you are, the less likelihood there is of you causing some unintentional dental damage.
Good luck, and I hope at least some of this helps 