I can imagine that this is a huge surprise, but congratulations.
I am not (yet?) a med student, so I cannot offer advice on handling med school (maybe you should ask this over on one of the forums for med students, rather than asking us pre-meds who haven't been there yet?). However, I definitely agree with those who said that you still have plenty of time to pursue your dreams. It's becoming so common for non-traditional students to get started later in life, you're still ahead of the game even if you do decide to take time off to focus on your child.
If you're in need of practical assistance, you might want to consult one of these organizations for some help with finances, a place to live, work, etc. and, *if you are considering adoption*, information about adoption:
http://www.nurturingnetwork.org/ (phone: 1-800-TNN-4MOM)
http://www.birthright.org/ (phone: 1-800-550-4900)
http://www.catholiccharitiesusa.org/programs/ (1-800-CARE-002)
http://www.bethany.org/ (1-800-BETHANY)
However, I want to emphasize that I don't think anyone--and I do mean anyone, including family members or friends!--has a right to tell you that you should have an abortion or place your child for adoption. I feel that too many women are pressured into making those decisions by people who have a knee-jerk reaction to single motherhood.
There are so many variables in life. The father of the baby might reconsider his feelings about being involved in the child's life once he sees an ultrasound or after the baby is born. Sometimes men have a hard time seeing the baby as something "real" (not just something to "get rid of") until they can actually see/touch it.
And on the flip side of the coin, even being married when you get pregnant is no assurance a man will always be around, with the divorce rate being as high as it is. Just as I would not tell a newly divorced woman to put her born children up for adoption, I would never tell a single pregnant woman to automatically choose adoption or abortion.
Nonetheless, IF you feel adoption is something you might want to consider, the below link is an article that discusses your RIGHTS as a birthmom...most importantly, the right to feel free to say no if you change your mind:
http://www.openadoption.org/bbetzen/parent.htm
And here is a site that outlines some of the choices you have in an adoption plan:
http://www.birthmother.com/adecisions_index.asp
I just wanted to try to offer some support.
My best wishes to you and your child.