Oh, dear, what a predicament to be in! I am so sorry! As a mother and a nurse, I can sympathize. I do work in Maternal/Child Nursing and do see a lot of teen pregnancies.
First of all, thank you for being "calm", I know you probably are feeling like you're "in shock". It is of utmost importance for you to be there to support her now more than ever. She is really going to need you...13 is soooo young to be having a baby and I can tell you, most of these young girls have no idea when they come in and they are laboring. It is really hard sometimes to labor w/them. Preparation and education is key at this point.
Second: Start prenatal care as soon as possible, she needs prenatal vitamins, and a good diet, possibly a visit w/a nutritionist.
Third: Without trying to sound "rude", once the reality of all this settles in, you should explore w/her options for the baby. Is adoption a possibility? Or will she live at home and have you and dad help her? I don't mean to sound indelicate, but, again, it is amazing how many people truly aren't prepared and just don't know what they're gonna do...
Good luck to you, this is not easy, and there are no easy answers. You will have to just figure out what is going to work for your family. Anymore, I think, well,pregnancy certainly isn't the worst thing that could happen....as hard as it is, there are worse things going on w/our kids these days. Good luck!