I'm 17 and my girlfriend just missed her period and we had sex the day she would be ovulating. I don't think i'm ready to be a dad but I don't want to be a dick and leave her.

What should I do?

You sure she's pregnant?

Yes

I'd advise a pregnancy test or at least one more month to confirm, but oh well
Let me just reassure you then.
Most people are never actually ready to be parents. They say they're ready only when life is stable (which doesn't happen that much btw). You can surf the internet about teenage pregnancy I guess.

I don't know if you have any religious affiliations or not but, either way, there are many organizations out there that will help in situations like yours. They can provide guidance, counseling, and even goods and monetary donations. It may not be your plan, but it doesn't mean your life is ruined.