If your mom makes an appointment for you... She still can't make you do it. If she forces you to go, tell the Dr. / nurse / counsellor (some laws require them before an abortion) that you don't want it to happen, that you're being forced. They cannot legally force you to abort if you say no, no matter what your mother says.
I think you should find your local pregnancy crisis centre and ask them for help. They're generally pro-life organisations dedicated to supporting mothers-to-be who are in need. Try
http://www.Birthright.Org/ to see if there's one in your area.... Or just google it. Some can even set you up in temp homes from teen moms if you find you really can't stay at home. But I hope it dosen't come to that.
As for your bf joining the army..... That's a wonderful sacrifice for him to want to make for you and baby, but it's also a huge commitment. And since you're underage, would you have to marry him before you recieved benefits? Would your mom allow you to? Were you ready to marry him *now* before you got pregnant? You still have time to think about these things, and while it's wonderful that he wants to make that sacrifice for you and his child, just make sure it's well throught out first!
Best of luck,
laura