There are a few things you could try to solve the issue of not knowing what to say in a conversation:
1. Find a topic that interests you and you are comfortable with. If anything is engaging to you, chances are it might be to the other person as well. 
2. Let the other person talk. Try to ask them open-ended questions about themselves. People often like talking about themselves and this will make them view you as a good listener. 
3. Try not to force a conversation. Silence is only awkward if it is perceived so. It is completely fine to not say anything for a few moments.
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