My gf is pregnant, I am not ready, what do I do?
You own up to the responsibility you chose to accept by engaging in sexual activity with substantial pregnancy risk. It may have been you two used protection and it failed; you might have failed to use protection altogether. Either way, stuff happens, and part of being ready for sex is accepting that even the best protection isn't foolproof, and having a plan in place. It takes two people to make a baby; too often it's only one who has to deal with the bulk of the consequences.
What you can do right now is support your girlfriend in whatever choice she makes. Her choices are to carry the baby to term and parent, place the baby into an adoptive home after birth, or terminate the pregnancy. You cannot and should not make the choice for her, but you can be honest about what sort of assistance and support she can expect from you with each choice to factor in to her decision.
For example, she needs to know if you have the funds to help her pay for an abortion, the commitment and readiness to parent, your feelings on placing the baby for adoption. Dishonesty and idealism screws a lot of young mothers very quickly per their partners pledging parenting support, money, or other help, then failing to follow through, and leaving them to deal with everything alone. So, don't promise to be a good dad if you're not sure it's what you want, don't have her wait for months while you rake up the cash for an abortion.. I think you get the idea!
So, step up to the plate, really suss out where you are in your life, what you can offer her and a potential child, and be honest with yourself and her about your goals, and how each choice will affect them.
Check out these articles for a bit more about parenting preparation and more information on various options: