I usually just say my piece on whatever people have been talking about. The other day Cherry was talking about her weird Harry Potter dream so I basically just responded how weird it was. You don't have to post anything super long, just get in on the conversation and let other people lead where it goes if you don't know what to talk about.
Sounds good. Just remember its not like you're signing a contract or anything, you can come chat for a bit and then if you don't like it you aren't obligated to come back or anything.