Tactics Necking with tongues is a tricky business.
On the one hand, it's one of the most exciting things you can get away with in public. Once you've got into a rhythm, gently slide your tongue into her mouth. Go easy at first, stroke her tongue using gentle, slow movements. You can turn up the heat once you get into it. If she stiffens up, or pulls back, she probably wants to go back to just kissing.