Well, its best to start off slow...don't just jam your tongue in there and start swirlin' around. Start off with soft kisses. Then ease into open-mouthed kisses WITHOUT tongue. While you're doing this, feel free to stroke his hair, or run your hands along his neck/back to make the experience more sensual. After a while, open your mouth a little more and gently caress his tongue with yours. You can do circular motions, side to side motions, or just gently flick your tongue against his. don't stick to one technique so that it doesn't get boring. You can switch back and forth between open-mouthed kisses and then back to using tongue.
From personal experience, ive noticed that some guys like when you gently suck on their bottom lip for a bit. Be careful not to bite, although a little nibbling can be sexy.
If its your first time, try to avoid making out while you are on top of him because it may cause you to drool in his mouth. Also, don't make those porn-star moaning noises unless you're trying to excite him to the point where he wants to go further.
good luck!
