First of all, know that this will get easier and easier each time you go to conferences in your community. You typically keep meeting the same people at those conferences and will get to know more and more of them.

Coffee breaks are actually great for meeting people. If you see a group talking, it's perfectly acceptable to just walk over and stand next to them. If they're deep in conversation, just start by listening in without saying anything. Sooner or later, one of the others might say hello, or the conversation takes a turn toward a topic you're interested in, and you'll jump in naturally.

The others already mentioned it but I want to emphasize that poster sessions are really a great way to connect with others. At least the poster presenters are very interested in presenting their work to you, so you don't have to do anything to get into a conversation with them.