Don't post "will you marrie me" on a bilboard or have it written by a skywriter unless you spell it correctly
I wish the best of luck to both of you. How far along is she? Would you marry her if she weren't pregnant? I think you have a very important decision to make but I guess part of that decision was made when you had unprotected sex.
As far as how to ask her? My ex had me close my eyes and hold out my hands. Then he placed a box in my hands and when I opened my eyes he said"so, what do ya think?" ----Don't do that, very unromantic. If the pregnancy is in fact confirmed, how about tying a ring to a small stuffed animal with a sweet poem attached. But definitely get on one knee, grab her hand,tell her how much you love her and that you would be honored to be her faithful husband and dedicated father to your child.