I have trouble talking to girls. Its not so much approaching them, its starting or keeping up a conversation. Any tips?

It's good you have no trouble approaching them. Starting and keeping a conversation... Well, simply start talking about something you have in common. Ask questions if you want answers. If she's interested, she'll ask questions too and the conversation will follow. Yet, if you feel like you're talking too much or you feel like you're talking to a wall, it's not a good sign, obviously.

But usually, asking questions will help you have her talking to you. If she doesn't answer, don't even bother to continue talking to her. As of topics to talk about, it's definitely easier to talk about things you have in common. For instance, if she's in your school or you're working at the same place, just ask something about a course/teacher or about a specific thing. I don't know what exactly; you should easily figure something out. Then, give some comments, share your opinion, etc. Have her asking you questions, too.

There you go.
Hope this helps a little bit. Let me know if it 'works'. :)