Don't tell her parents. It's quite obvious that you guys want to be responsible. That's quite enough. It's probably better if her parents don't know. Specialy if they are strict. And it really won't solve anything.
First and foremost, use condoms. Just buy them. If you don't know how to use them then look inside the box and then practice.
Check out planned parenthood. I think its
http://www.plannedparenthood.com
I don't know the exact laws wich probably vary state to state, but they will give you all the info you need about birth control including cheap morning after pills. Get one and keep it on hand.
If your girl is old enough to be sexually active she should also be going to the gynocologist. Tell her to tell her mom she needs to go. Simple as that.
She can discuss birthcontrol with her doctor and it should remain confidental (though maybe there are laws conerning this?). Then all you have to do is figure out how to pay for the presrciption. Depending on what she gets it is relatively cheep.
edit: This made me horny
How pregnancy happens animation