Approach and start talking. Yeah, it's intimidating, but there are a number of well-known small talk subjects that you can start with, especially the weather. And since you are uncomfortable talking to normal strangers, you can practice your small talk on normal strangers.
Then work your way up to talking to semi-attractive women working in retail. Then finally try approaching this girl at work. Only, I think that you should approach her at the bus stop. Start a conversation shortly before the bus arrives, and then sit down next to her so you can continue the conversation.
Is she seeing someone? Two ways I use to find out. One is to do some flirting during the conversation. If she is seeing somebody and/or uncomfortable with your advances, she will eventually mention the boyfriend. A more indirect way is to talk about current movies. If she is interested in you, she will become enthusiastic about some upcoming movie that you are talking about, to hint that you could ask her out.
Good decisions come from experience. Experience comes from bad decisions.