Pick any app or YouTube channel, experiment with different guided meditation videos to figure out what works best for you. My simple suggestion would be to get started with meditations that make you focus on your incoming and outgoing breath. Continue doing it twice thrice a day for 15-30 minutes and slowly increase the duration to an hour per sitting. Might take you weeks and months to do it or simply join a meditation course (easiest way to quickly learn meditation) It’s no magic but it’ll help you be calmer as a person - in turn, positively affecting your speech as well