Alrighty, my first advice would be to see how she feels about alternative options. I don't want to start another abortion debate here, but it's certainly on the table, yes? Heck, her parents might even nudge her in that direction. I know mine would.
Second, is she the type to "test" her man a lot? Most 19 year olds are. The "I'm late" schtick is the MOAB of shit tests in the female arsenal. So it might turn out to be a false alarm. Also, the child -might- not be yours. Again, she's nineteen. They have confidential DNA tests - you send in a bit of your hair and a bit of the baby's hair to this lab. It's not an "official" result but it can pave the way for an official DNA test if the results seem funny. You only have a year-ish to contest paternity.
(As if the happy dads commenting here don't hate me enough already) Don't rush into a marriage for the sake of the kid. I've seen that make things worse for a lot of guys. Take responsibility for the child, yes, but remember you've only known the mom for 4 months.