there could be any number of causes. 1. Maybe she's nervous and can't relax... that's a big one. I'd suggest a shot of your favorite liquour before starting

2. She's not lubricated. Diablo is right on the money, just use some saliva or even lube and that may help. 3. You're using your finger wrong. Some guys just don't know and they start sticking their finger where the pee comes out... that's not very comfortable to say the least.
If your girl has never used tampons or masturbates it will take some time. Just be gentle with her and let her get more comfortable. Even going down on her may loosen her up some.
Talk it out with her, let her guide your hand. I've been told by guys that it's really arousing when I take control
