Well, 12 is a little too young to be kissing, IMO...however, if you insist upon kissing (it should be your decision as well...if it's her pressuring you...I tell ya...move on)...get in a situation where you're sitting with her or something, and kind of "lean" into her and if she starts leaning in as well, pucker up.
It's not the big deal people make it out to be. I didn't get my first kiss until I was 20 (dating kiss anyway...parent's always kissed my cheek), and he's now my husband of 13 years.
It's second nature...don't stew on it and 2ND nature should come through.