What should I say to my father for the first time, we have never met or talked before?

[Cassandra]

I'm 19 and I have never met or talked to my biological father before. My mom had me at 17 and it was a friends w/ benefits type of thing and when she found out she was pregnant she wanted nothing to do with him and never told him she was having his baby, but she said people in their group of friends did. When I turned 18 my mom told me his name and showed me his Facebook and I sent a friend request and after a few months it was never accepted. Then, just a few days ago I received a friend request from him.
I accepted his friend request and he hasn't messaged me or anything. I'm almost positive he knows I'm his daughter, considering he can see I have the same last name as my mother and he probably sees her posts on my facebook wall. And if he's looked at my pictures I'm sure he's noticed how much we look alike.

Like I said, he hasn't tried to contact me yet. I'm not sure if I should message him first or wait it out. If I do message him, I have no idea what I should say. Any tips or suggestions? Would be greatly appreciated