So, I've never been kissed, kinda.
Guys have tried to kiss me, one even almost did, but I've denied them because I tell them that I don't want to rush into things, even though I totally do!
I'm just afraid. Where the heck do my lips go? What's the difference between making out and french kissing? When there's tongue involved what do your lips do? WHAT DO YOU SAY AFTER YOU KISS!?
I'm scared I'll be a terrible kisser and things will get awkward. I'm scared that I'll move my lips the wrong way. I'm scared that my breath will be bad. I'm scared I'll start breathing heavier cause I'm so nervous. I'm scared that I'm 18 years old and I've never been kissed.
O O, (No Xs cause, ya know, NBK ;)
I've been delegated to answer these questions, possibly because if anybody would have messed up a first kiss, it would have been me. I was so awkward in high school that I regularly fell up stairs, and I was fairly sure that my first kiss would involve me missing the girl's entire head and toppling off a cliff that had not previously even been there. But I didn't, because kissing is practically an automatic thing that your mouth does when confronted with a hot person; at least, it is if you can convince yourself not to stress about it.
"I tell them that I don't want to rush into things, even though I totally do!"
This is completely normal. I always used to interrupt romantic moments with "...SO ANYWAY here is a story about my cool dog" because I was worried about screwing up the inevitable outcome of those romantic moments. The fact that you do this is not weird, and you'll stop once you finally get this out of the way and discover that it is not actually terrible or complicated.
"Where the heck do my lips go?"
On the guy's eyeball.
They go on his lips. Trust me, this will all feel more obvious once you have successfully kissed an actual person on his actual mouth, and are like "Ohhhh, that was easy." I will come back to this because you mentioned it twice!
"What's the difference between making out and french kissing?"
Everyone everywhere has different terms for everything, but in my experience making out just means "open-mouth kissing for some period of time," while "French kissing" specifically means "kissing with tongues." It is so named because English people pretty much used "French" as a synonym for "sextacular."
"WHAT DO YOU SAY AFTER YOU KISS!?"
"Nice mouth, idiot! LOL."
No, seriously, you can answer this better than I can. What did you say before you kissed? Who is this guy (or gal) to you? Are you ~totes in lurve~ or just messing around? All of this changes the situation, which is why I can't just give you the One Weird Old Thing To Say After Kissing. Often you can just look into the other person's eyes in silence, and then maybe kiss again, like a robot whose programming is >10 KISS >20 GOTO 10. But if you really must break the silence and have no earthly idea what to say, then fine, you twisted my arm: say "You're a great kisser." Unless he spent the makeout bashing your forehead with his dumb mouth, in which case he will know this is a lie. Otherwise, everyone likes to hear this kind of comment.
"When there's tongue involved what do your lips do? ...I'm scared that I'll move my lips the wrong way."
Dude. Dude. (I know you're not a dude but there's no female equivalent for just looking sternly at somebody and saying "Dude.")
There is no "wrong" way. You connect mouths and make the very same kissing motion that you are picturing right now. If you do this chastely, lips mostly closed, that's fine! If you feel comfortable with tongue involvement, go for it. If you're really worried about something doing something completely terrible and wrong, then just ease into it and just react to what the other person is doing. The one thing I can think to caution you against is jamming your tongue violently into his mouth, but that's more of a dude mistake, and I doubt it's something you need to be warned about.
"I'm scared that my breath will be bad. I'm scared I'll start breathing heavier cause I'm so nervous."
Breathing heavily is associated with basically every hot thing that humans do, and I cannot possible imagine a scenario where a guy goes "This girl is breathing heavily. Yuck!" This is a passionate thing and therefore the exact opposite of something to be worried about. Also: carry breath mints, but the dude is going to have worse breath than you anyway, because we pretty much just eat whatever we find lying around.
"I'm scared that I'm 18 years old and I've never been kissed."
I promise you that this is the kind of thing that happens instinctively as soon as you're unafraid of it enough to let it happen. But if none of this advice has been specific enough for you, then try this: hit up YouTube and look up kissy scenes from movies you've liked. You can mine those scenes for the particular details you want, but all you're probably going to see is two people connecting mouths and then staring longingly into one another's eyes. Nobody says the secret password once the kiss is over; nobody goes "Well, I love you, but your lip technique is unacceptable, relationship over." If you're still convinced that you're going to be awkward and terrible at this (even though I promise that you won't), just be mostly passive and mimic what the dude does. Most guys won't notice, because their train of thought will pretty much be "I am kissing a girl, aw yessss."