Yep, I agree with zro and lilwing, you either have trust issues or you have reason to believe that your gf is too immature and not ready for fingering. So I recommend you either spend more time together and talk about sexual activities so your more aware of how each other feels towards the them, or break up with this girl. But doesn't sound like your ready if you're that worried about your gf reacting the wrong way and calling the cops on you.
If however, you ignore this advice and want to go ahead with it anyway I recommend you follow Charlie Boy II's list of instructions.
Seriously though sounds like neither you nor your gf know each other well enough or are not yet mature enough for this type of activity. Especially don't do it if you think your gf isn't ready or doesn't really want you to.
"Every once in a while, declare peace. It confuses the hell out of your enemies."
-Rule of Acquisition 76.