This might be a case of the blind leading the blind, as I'm rubbish at social skills myself, but "Hi, how are you?" is probably a good start. You could ask them what they've been doing recently. Ask them questions about themselves, tell them a bit about you. See if you've got any common interests, and if so talk to them about whatever the common interest is. Talk about what's happening in the news. Talk about things that are particularly relevant to the time of year, eg Christmas in December, or holidays in the summer. If all else fails, you can always talk to them about the weather. There's lots of things you could talk about.