Plz help i kissed my best friend and the girl who likes him is gonna bash me up?

Explain it to him, and see if he can get this girl to back off. If he really cares about you, he may even say he'll dump her if she has a go at you. He should sympathize, and if he doesn't I'd drop him and get over him, though that's not easy. Otherwise, stay with him, show him that he can't go out with one while lusting over another. Get him to make up his mind. And in the meantime, stay with friends who will support you if you get challenged by this popular girl.