How do I kiss my girlfriend?

Just tell her you don't know if you're good at kissing, or not, because, basically, you're never kissed anyone in a romantic way, but that you'd like to try kissing her because you think she's really special, and would she mind giving you some pointers. Or ask if she'd like to practice kissing with you. Damn, you gotta start somewhere, right? So, just pucker up and go for it. Later on you can slip in a little tongue action. Good Luck.

try it first to kiss your fingers.

Quit worrying so much. When the time comes, you'll figure it out. Just don't be in a rush or hurry. Getting in a hurry can hurt you both. Go slowly.