Well, first off, you should probably locate the chat if you haven't already done that. Just simply look at the right side of the screen, and it should be there. It's a little box that says live chat above it.
Next thing, figure out a message. Normally, if your new, a little hello, how's it going should work. Type that in the box below (it should be completely white.
Last, hit enter or return (depending on the computer your using.)
A final note- if your using an ipod touch, iphone, or any other mobile device, I recommend you don't try chatting. It's plausible, but often times it involves hitting refresh a bunch of times.