Is it normal i have no idea how to break up with her...

I'm a female and I have some advice,

1. Tell her you don't want to be in a relationship. It's that simple. If she really cares about you, you guys will remain friends.
If her best friend gets mad, who cares? It's just between you and her, not her best friend.

Best of luck mate.

As i have said in a post before. Anything but a text

First off, please do not use the dreadful word, or it's abbreviated form, especially at the beginning of a sentence. This word is of uncertain Choctaw or Bantu origin, and is derived from 'Oll Korrect' or 'Old Kinderhook'. It is superfluous, meaning that it is not needed or unnecessary, and it is often confusing or conveys a mediocre quality to the reader.
In this, "About two weeks ago, a good friend of mine invited me to my first party." This would have been preferable and satisfactory.

As for your problem, I agree with the other comments, above.
However, I would make an ample opportunity to converse with Lexi. I think that you need to ask yourself, why you are not ready for a sexual relationship, as yet; something that you didn't elucidate on, in this. 'Just because' and 'I don't want to', are not reasons.
Then, sit down with Lexi, and explain the real reasons, truthfully, why you don't feel ready (write these down, if you need to). Next, explain that you would like to remain on friendly terms with her and her other friends. Then, you will need to negotiate, what constitutes a 'friendly' relationship, rather than a 'sexual' relationship. (Which behaviours and acts are mutually acceptable).

I agree with NeonLighterz. But if she asks about the party, like "well, what was that night at the party all about then?", just tell her that when you both hooked up the night of the party, you were following what you felt in that moment. But as time has gone by since then, you feel like things will be better off if you two are just friends.

Or email

Text message.

If she likes you it will hurt her a bit, but ur just a teen... its gonna have to happen at some point!!!! are u just gonna stay in it until she asks u to ask for her hand in marriage???? END IT. feelings can be repaired.

That is a good question, and one for which I have no answer.

It's going to cause hurt no matter what you do at this point. Just be honest and deal with the fallout. It will pass eventually. It's better to go ahead and get it over with before it goes any further. The longer you wait the more hurt it will cause.

Due to your age she will take it wrong no matter what you do or say so honesty is your best bet right now. As you both get older it won't matter any more because she will understand how silly she was.

As a female, If you said, "I like you, but I just don't want to be in a relationship right now," I would probably take that as a rejection. As much as I hate to say it, I agree with douchebag9, you should think about why you feel this way. If it's just that you don't want to be tied down, that is one thing. If there is something else, whatever it is, I bet Lexi would be a good confidant. She'd probably be flattered for you to share something personal, even if you don't want a romantic relationship. I had a boyfriend (briefly) in high school that had issues because his mom left. It happens.

Your friends congradulated you for making out, what are in middle school ? Just tell this chick straight out you want to be friends and you're not looking for a relationship..

DEVOUR HER.