My girlfriend and I went through something similar a year and a half ago. Accidents can happen regardless of whatever safe guards you take. Although it seems that you reall need to improve on this in the future.
Reading over a lot of the replies, it seems the general consensus is that you need to be responsible and mature. Regardless of however you feel about the situation, you need to meet and talk with her face to face because I can tell you with absolute certainity that she is much more affected by this both physically and mentally than you are. You need to find out what she wants to do. If she wants to keep it then thats that.
In my situation, both my girlfriend and I realized that we weren't ready to be parents. We were both 24 years old and wanted to do more with our lives than be parents at the time. Additionally we could barely afford to support ourselves let alone a child. Neither of us wanted or were ready for the responsibility of a child.
We both decided that an abortion was the right thing for us. Some people would find this reprehensible and that is their right, but in this country abortion is a legal option. We are both biologists and know the difference between a group of undifferentiated cells and a living breathing child. If this is what you chose then I would strongly suggest contacting your local planned parenthood. I found their website informative and more importantly their staff was knowledgable, courteous and NOT JUDGEMENTAL.
The only judgemental and hateful person we encountered was the Right to Life wackjob protesting outside and handing out pamphlets with graphic pictures of partial birth abortions to the women entering and leaving the clinic.
You're not going to be having the abortion but you had better be there for that girl if she decides to have one. If she needs you to drive her there, sit with her in the waiting room, take her home, and comfort her, then you better be there for her. Expect to pay the bill too. Try to be there for her throughout this, and you will never ever be in this situation again because this was one of the most difficult things I've ever gone through and never want to go through again. The Planned Parenthood people also will give you both counseling on birth control because they don't want you to come back and have to do this again.
With all that said, I don't regret the decision we made. It was a joint decision and I think it worked out for the best. We are still together, doing well, and even thinking about buying a house. It brought us together and showed us that if we could go through that we could anything. I was raised Catholic and went to Catholic school and even Catholic college, and while I sometimes wonder what would have been, I also wonder how I would have stood up to being a 24 year old father who couldn't afford to move out of his parents house.
You may feel like you're alone but keep in mind this statistic: 1 in 3 women in the US have had an abortion.