Several people below recommend toastmasters. Do it. https://www.toastmasters.org/find-a-club
They have been around for a hundred years, because they are good at it. There's nothing wrong with other approaches, but toastmasters is good at making nervous people comfortable. Toastmasters' meetings have short, improv speeches every meeting, so you can practice the thing you need work on. And you can get feedback, delivered kindly!
Improv is fine, too. I just find it a little scarier to get started in. And it doesn't have as much emphasis on "organize your thoughts". Of course, it's a lot of fun!!
Anything that gives you practice on what you aren't good at is good. Give lightning talks at small users groups. Hell, get a weekend job at Starbucks. Have a friend ask you questions like this, and answer them on your feet. Get up in the morning, and explain something to your mirror. Find the nearest small group of people, and summarize the responses given here!
Practice, practice, practice