there is a lot of good sense in these responses, particularly the ones that are obviously from guys. This is what most of us guys have struggled with and still struggle with. How to get a girl talking to you. What to talk about and how to stop yourself puking up with nerves.
If you are not confident enough to go over and sit next to him, "is this seat taken?" or ask if you can buy him a drink then some subterfuge is called for. The one that really makes me smile is to "accidently" knock a drink on him if you are in a bar. If it is a coffee bar then this might not work the way you want it to. But in a bar, you knock his drink over him, or spill yours on him, and then really go overboard with your apology, making it last and last and last. Offer to replace the drink, get some towels, pay for dry cleaning, and keep on apologising until he softens and says "its okay, it's no big deal".
Then you have to make conversation with him so if you don't know how to make conversation then this won't work but it is a great way to break the ice. And even if the conversation doesn't develop at that point (because they are his favourite trousers, shoes etc) then he will remember next time you see him and you can go over and apologise all over again and offer to buy him a drink as a peace offering.
Give it a go