First you have to get comfortable being on stage. Do some theater or singing or something where you're using someone else's words. Read speeches by Thucydides, Churchill, Lincoln, FDR, MLK.

Then move on to saying your own words, like improv or join toastmasters.

Do some physical training to improve your ability to stand correctly, could be tai chi or yoga or pilates or karate (the key is to find a good instructor not any particular discipline). Keep your grooming up to date.

Listen more than you talk.

Watch people whose style you admire that would feel authentic to you, steal liberally from them.