Hold her (Tip for guys)

Guys, you aren't the only ones that worry. Girls freak out over this sort of thing. Make sure your a kiss worth remembering by keeping her comfortable and calm.

Hold her hand or pull her into your lap/arms so that she feels secure.

If she's still nervous stroke her cheek or kiss her forehead and say something to calm you both down.

Once she's calm(ish) tell her to close her eyes and them make sure your just a few centimeters from her lips, and whisper "Trust me." it's amazing how calm that can make her.

Then kiss her gently and move back. If she's comfortable try a few more soft kisses before moving on to the trickier stuff.