Don't take things too seriously. This includes not taking yourself too seriously.
Start the conversation. When you begin the conversation you take the pressure off the other person. You might need to force yourself to do it but it gets easier and more natural. You could start with something as simple as: "Where are you from?" or "I love your jacket, where'd you get it?"
Know what's going on. The more you know about what's going on in the world the better your chances are of finding something to talk about. In conversation be prepared to talk about something you found interesting, start with something like: "I saw the funniest thing yesterday..." or "Did you see that article about...?"
Take a sincere interest in the other person. It's not about you. People are most often drawn to people who make them feel comfortable and show a real, genuine interest in them.
Learn to read body language. Look for non-verbal cues. You don't want to be that person who traps someone in a conversation or the last one at the party who hasn't picked up on the fact that it's time to go.
Take a friend. Entering into a new social situation is always easier if you go with a friend. It helps to know someone who likes you, is there with you.