Read around some of the old threads on here mate.
2-useful-conversational-techniques-vt12 ... highlight=
This is a recent one started by Ryan that I liked a lot. There was also a thread I read a while back (and I'm sorry but I can neither remember who wrote it, nor can be bothered to go back and find out who it was), who advised some techniques to help you get better at this sort of stuff. One of the techniques was to just pick a random object up and start writing about it. Just write whatever comes into your head. And various things like that.
Conversations should be the easiest thing in the world - how many people do you see/hear having conversations about completely trivial and random things - but so many of us struggle to get those thoughts to pop into our head at the right times. Those two threads I'm talking about should help you start to change that, and there are plenty of others around as well!