Give yourself time. A baby is a very big thing to accept in your life, especially if it's not planned, and it's really a normal thing to have misgivings and be afraid.
If you are just thinking that she MIGHT be pregnant, don't worry too hard about it until you find out that she is for sure. You wouldn't want to worry about something that isn't taking place.
If she is pregnant, you should talk to both yours and your gf's parents together and tell them your wishes, which they should respect, seeing that she will not have the baby until after she graduates and you are almost a "completely legal" adult. Hopefully they will be able to help you both out in some respects. Telling the parents early on will ensure a greater possibility that you and them have a mutual trust and respect for each other.
If you are both ready to love and care for this baby, that's all you need to be good parents. Every time I get pregnant I'm scared too and this is my 7th pregnancy and 6th full term pregnancy and I've been married for nearly 10 years. You'd think it was old hat for someone like me, but each baby is a change in my life and there are many things to consider. It is the responsibility that you are realizing, but this is very good that you know that it is there, because this is no small thing: a human life.
The only thing that you would need otherwise is the finances, and there are programs to help if you don't quite have the means to do it. Otherwise, I'm guessing that you are a working guy, so you should start making more money to provide for the little one, because it isn't going to be cheap.
Be a good and supportive partner for your GF. She wants the baby and you wanting it too is the best thing for her. You have 9 months of pregnancy to come to love this baby growing inside of her and also that is enough time for you and her to consider that if you can't handle it, there is always adoption.
I wish you all the best