Consider your enjoyment

If you don't enjoy sucking cock? Quit it. Stop right now. Stop reading. Don't bother.
There's one secret to giving a good blowjob and it is NOT a skill
You have to enjoy it.
If you can't cover that requirement - you have no business sucking dick.

Base exploration

You start with just the tip of your tongue and focus. Focus on his bodily responses, his breathing and his sounds. Slide that tip along the length of his shaft, slowly over the tip and subtly past the edge of it. Feel exactly when he twitches, when he shuffles or presses up against you, or moans. Repeat the movement while increasing pressure and flattening your tongue against him. And then repeat it again even harder, all the while registering his every motion. This should already give you an indication of the parts of his cock that are responsive and which parts are sensitive. Crucial knowledge.
Bonus tip: When tasting him on just the tip of your tongue, you're going to savor his salty tastiness. Exploring the tip might reward you with a drop of precum - an achievement in and of itself. Plus, feeling a cock twitch against your tongue as if it has a will of its own, and feeling the power of being the one eliciting these responses - YUMMY!

Getting it wet

There's nothing as bad a dry and chafing blow (or hand-) job. Getting fucked by sandpaper is never a recommended feature, so make sure to build up an appreciation for you naturally packed lube: saliva. After that first test you're going to slip him into your mouth. Fold your lips around him, and slide down to the base while swirling your tongue across his entire length. Getting him nice and wet while slowly bobbing up and down. And all the while still paying attention to his every move, and that delicious taste in your mouth.
Bonus tip: That first contact - that first time you swallow his dick is when you truly get to appreciate the gift he's allowing you - worshipping the thing that is most important and dominant in all his daily thinking. A vulnerability and constant source of insecurity - yet you get to touch, taste and suck it. YAY. Plus - feeling him harden in your mouth? Epic turn-on (to me, anyway)

It's a ball-game

When you're getting your sloppy on with number 2 - let you fingertips trace softly down and over his balls, just barely touching the skin before locking fingers around his balls. Slow and gentle, don't wanna send him through the roof if he's one of those no-no balls types. It's an easy test, because you're either going to get a very clear HELLS NO response or a positive reaction - which you can then follow up with your tongue and lips. Licking and sucking them just to determine his ballsy needs. Some like all of it, some dont want you anywhere near them - it's best to quickly find out which camp they're in.
They're balls. This is a pro-active service. A work of love. But they're still balls.

By now you should be the proud owner of a rock-hard cock and a horny-as-fuck dude, eagerly awaiting your attentions. So now you get to work on the head of that throbbing cock. Test just HOW sensitive that erogenous zone is, and how much you can make him jump. Wet your thumb (like, get it wet wet) and run it over the tip. Dip the edge of a nail in the ridge at the very top and watch him squirm. Run it across that ridge that's somehow extra sensitive and watch his delight (or stop, if he's panicking, obviously, some blokes are TOO sensitive there). Circle that tip slowly (much like you would with your tongue when you're sucking him down) and maintain eye-contact to gauge all of his responses and drive him crazy with your intent. And when your satisfied with the knowledge - slide your lips around him and start sucking, again.
Bonus tip: There's something amazingly powerful about moving your thumb ever so slightly but eliciting a body-wide response. With his hips thrusting, dick twitching and eyes widening. The slack jaw, the moans. It's delicious.

Pressure

When it comes to sucking cock - all you need to understand is that you have a couple of tools, all of which you can use with a certain pressure and intensity and speed. Your tongue, your lips/throat and your fingers.
How hard you're going to grip him with your lips, how hard you're going to suck and pull him in and how tight you're going to grab on to him with fingers - is entirely dependent on the suckee in question. Their preference should be your entire guidance. Everything you think you know about sucking dick can be abandoned at this point - all you have to do is ride his queues. You should (after executing 1 through 4) have an indication of his preferences. Now you're going to experiment. Blow him from a slow pace to ever increasing (but steady) speeds. Keep the tempo the same though - go slowslowslow or fastfastfast but not slowfastslow or variations - men are simple creatures and like predictable movements. Allow yourself to linger when you find a pace that works and switch when you feel his body getting limp or distracted. This is when the real sucking begins. And continues. Till the end.

Depth and the power of the roof of the mouth

The roof of your mouth is designed like a sextoy for dudes. It's ribbed and textured for his pleasure, even if it's hiding between a row of teeth. It is now your challenge to run his tip against it, press it in there and allow the contours to caress his glans. Take him in deep, press him up in your mouth and then pauze entirely. Press him up harder with your tongue and then make a swallow motion a couple of times. See him perish in shock and do his best to hold on to not cumming. Almost always a guaranteed success.
After that - well - it's rinse and repeat. You'll get to work up a cramp while mixing 1 through up into an intoxicating mix of pleasure. All of it based on his responses and your own creativity. Be sure to massage, knead and pinch his inner thighs while you're sucking, or press his pelvis down into the mattress when he bucks up. Countering his pressure is hot as fuck.

And when you wanna get really rowdy - try some out of the box things. Run your nails past his shaft. Let him feel your teeth on that ridge on the tip (consciously, not accidentally!). Use your hands in unison with your mouth or to play with his balls. Fold both hands around his shaft en wiggle your fingers on the shaft while you swirl around his tip. Gag yourself on his length and then stick out your tongue as far as possible. See if you can touch his balls while he's in the back of your throat. Drool, swallow, lick and caress to your hearts delight and just enjoy his enjoyment.

But most of all: just have fucking fun with it!
Blowjobs are powerful - yet ultimate submission at the same time. You're in control but at the same time fully dependent on his needs and whims. You can't 'win' if you don't keep to his rules and he can't cum if you don't read him right. It's an intimate dance and it's fucking awesome. Blowjobs rule.

SO GET ON THAT!