People love talking about themselves. A simple "how are you today?" can invite them to go into detail about themselves, which takes the pressure off of you to do the talking. Also, pay attention to current events. If something major is going on in the area around you, it is likely that people will want to talk about it. Make a small comment on the weather, traffic, sporting events, etc., and you will likely engage the interest of those you are trying to talk with.
It's also important to know when people just want to be left alone, especially when they are at a restaurant. Pay attention to facial expression and body language. If people turn away from you, frown, or give you short, clipped answers, they probably want to be left alone and enjoy their coffee.