How do I start a conversation with my co-workers?

I have been working at McDonald's for about two months. I don't really talk to anyone, but this one lady and a guy who's a little younger than me. I don't know what to say or what to talk about when I'm around my other coworkers. They think that I'm shy and that I don't like them (which is half true). I don't like them much. I'm not shy, but it does take me a while to let my guard down and open up. They keep asking me why I don't talk. All they really do is talk/ mostly complain about work and other co-workers. I don't have problems with anyone, so I have nothing to complain about. Most of them have kids even some of the young ones who are my age (19). One girl who is 19 or 20 has two kids! And another is pregnant at 19, living with her grandmother! I don't have kids, so that's another thing I don't have in common with them. Also there is a girl who is crazy. She admits that she's crazy and tells the blunt truth. She tells people what she really thinks of them. To be honest they're loud and ignorant; I don't think any of them go to or have gone to college, but some of them are in high school. Please help figure out what I should talk to them about! Because I'm really getting sick of them bothering me about why I don't speak or if I'm shy. Please help! 10 points to the person with the best advice/answer!