I'm a little unclear as to what you mean - do you mean you have a bit of trouble thinking of things to say when you're with other people? If so, I think I'd just say something like:
"I'm just not a very talkative person. I prefer to listen to other people."
(Sorts the problem out neatly, so long as you sound reasonably happy when you say it. It's meant to be a 'cut-off' statement, but try not to make it sound sulky otherwise it'll make others want to stay away).
"Oh, you wouldn't want to listen to me!"
(Self-depreceating, may prompt them to ask you open questions and help you to talk more. May or may not be beneficial - judge for yourself).
If you don't want to talk about yourself / do most of the talking, just ask people open questions. This a) gets and keeps them talking so that you don't have to, and b) gives you access to information you can then use to be more talkative, either by asking them about how things are going next time or gossiping to others. "You know, a friend of mine said the other day that..."
Hope this helps.
