Firstly, recognize that learning to converse in a foreign language is a major undertaking. While its never too early to start, take it easy on yourself and realize that even with constant practice it will take years to really be able to say anything you want.
My strongest advice is to keep the conversation flowing. Nod along whether or not you fully understand what’s being said. When the conversation comes back to you, say something even if it creates a rather jarring transition. It’s okay to steer the conversation back toward things you do know how to say when it starts to veer in other directions. The longer you’re able to keep a conversation flowing the more confident you’ll become. Try to find A way to say whatever it is you want to say, no matter how roundabout.
You’ll make lots of mistakes. You’ll often be misunderstood or not understood at all. Deal with it. Just keep practicing: have informal conversations as often as possible and just accept that you won’t understand everything nor always be understood.
These prior replies are also relevant: