Put your hands on the back of her head or her neck, and kiss her.
Watch this: Back To The Future - Earth Angel Song
As with everything, you will get better with practice. :)
Here are some advices from http://www.forum.hr/showthread.php?t=465837 (in Croatian).
Don't go too fast or too strong, don't use too much of your tongue.
(by coconutmilk)
Be gentle
First, use your lips on her lips only. Then, add tongue. Don't put it all in her mouth, and don't get her wet with your saliva (can happen when you open your mouth too much). (by Rainbow Lady)
Let her do all the work. All you need to do is to react fast enough (to her moves). So if a girl presses her lips on your lips, and moves with tongue, you follow her lips. Her lips will open, so your lips will open. She goes slowly with her tongue, you go slowly with your tongue. Play. When she closes her mouth, and gives you a kiss, you do the same. So: just follow the girl, and her moves. (by phantom_fuxor)