1. How do you get good at giving talks?
2. How do you generate opportunities to talk?
Most of the other comments are focused on the first (practice), but if the question is the second, it is really a question about marketing.
You have to become famous in a niche, perhaps write something you can be known for. Try to develop a platform, or else write for a well known brand.
And, you don't necessarily have to be "famous". One can keep an eye on conferences or other gatherings at which one would be interested in speaking. When they announce request for proposals, submit one. Spend a few weeks working on the request and make sure it is really well thought out and interesting.
when you get an opportunity to speak, take it (no matter how small). Speaking is a good way to build up that "fame" rather than the other way around.