Do you mean when you're starting a conversation with someone you don't know so well?
In this case, it's very useful to try to find common ground. This is why starting a conversation by talking about the weather is very common - we can all see how it is, was and might be. This can lead to a conversation about past activities and future plans.
Just a simple thing like this can help to break the ice and make you more relaxed.
Also, try to understand the other person: What do they know already? What might they like to know? To what level will they understand what you want to tell them?
And remember, the responsibility isn't just on you to make the conversation. Listen to what the other person says and use there ideas to get things going.
You might feel nervous and this makes your mind go blank, but just starting by talking about something simple like the weather can really help you to relax and give you the confidence to move onto other topics.
Your situation is very common, but keep trying and you'll get better with practice)