I find that in the workplace there are two types of random conversations. One is the typical elevator/break room situation: you're stuck getting coffee or going to the 20th floor and someone says something to you. Normally it's something "funny", just laugh and play along. Say a follow up joke. Smile. Get away. The second is something along the lines of you've seen the person around and you're trying to get to know them better. Ask typical things like, "Where're you from? What college did you go to? What kind of music do you listen to?" ad naseum. You'll never know the types of people you meet. Every now and then there's someone who is pretty interesting. In less formal situations (school, clubs, college, etc), normally a compliment would be a good start (e.g., I like your jacket, where'd you get it?). Or whatever situation arises, poke fun at it. Don't just stand there and whip out the PDA. That just shows how unsocial you are.
Part of being human is being able to communicate [well]. If you can be comfortable in these types of social situations, more power to you. As you get older, you pick these things up more and you'll soon lose your anxiety. It's not like an interview or something. And if you don't want to talk, normally a smile would do, even if you don't mean it.
Jam it back in, in the dark.