I don't know what to talk about when I'm with people

[I miss you]

I want to make some friends that are girls and I have no problem approaching them. I'm shy but I still approach them and start a conversation and I normally start off with something like "Hey is it okay if I sit here?" (in a class room and no ones by her)

 

Sometimes people are next to her and I wouldn't know how to approach them.

 

It seems like the only time I know how to talk to a lady is when she's not busy or doing something. I could introduce myself but I never know what to say after that. If I knew what to say I think I could open up to them more... And possibly ask her out on a date.

 

What could I do to start conversations better and know what to say when she doesn't?