Hardest thing I've ever had to do is integrate into my step sons life. I find it easier being a dad than a step dad and that's no exaggeration.
It's taken around 5 years to get into a good place with him. He's now seven so not quite the same age bracket but same concept.
What do you talk about? You talk about their interests. Something that makes them comfortable.
Anytime I want to have a proper conversation with my step son I talk about super heroes (Batman, Flash, Hulk, Ironman etc). I can sit and talk for hours on end with him about them. I have the same interests or at least I did so that helps. I'm a massive football fan and like watching rugby however I can't talk to him about that because he just has no interest in them. The moment I mention them he switches off. So keep it to what they like.
I'd also advise taking an interest into their school life. Don't have to go heavy on this as it can be quite personal but you could talk about their favourite subjects and maybe offer help on homework/coursework.
Remember they too will have similar worries, some of which they may voice to their Mother which isn't a bad thing. Talking with your girlfriend is key. I was lucky to have a lot of support from my girlfriend and I can't thank her enough for that. She's your starting point to understanding her children and what their interests are.