Have you thought about why you can’t hold down a conversation? I don’t know much about you just by your question. But you may suffer from a simple lack of self-confidence or a severe case of shyness. There are ways to fix these problems.
Have you thought about seeing a counselor? Professionals are trained to tap into that part of the mind that says you are no good. They help people build the confidence that helps them succeed.
Other than that, you could do a lot on your own as well. Participate in group talks more often. And at the beginning, you don’t even need to talk much. A good talker is a good listener. So take interest in what other people have to say and listen to the words carefully. Too many people think that to be an excellent conversationalist, you have to dominate the entire conversation. That is absolutely not true. In fact, you are encouraged to let other people have their say. It shows that you are kind, considerate of others and not attention seeking. Also, by observing other people talking, you gradually get the sense of what it takes to be a good conversationalist.
So once you are more confident that you have learnt how to be a good conversationalist, practice holding conversations. You should ideally start doing this at your own home, so that the margin of error wouldn’t be so claustrophobic. This exercise requires you to write down topics of discussions along with possible responses. For one topic, write at least five ways that you can respond. Start with the topics you know very well. For a greater challenge, include a topic you are not familiar with or you may not feel comfortable talking about. A good conversationalist is able to talk his way around anything. He tries to convince people that he knows what he is talking about. Your goal is to persuade people to join your team and see life from your perspective.
After going through all these, I am sure you would have learnt how to be a good conversationalist!