Journalism

Hi Abbas,
The best way to start is to know what you are good at.
You could try to cast as many matches as possible, or be the most entertaining person on stream, but if you are not sure what your strengths are while doing a commentary, it would be preferred that you put in the effort to master one thing, than to do a lot more others at 50% capability.
There's a few requirements expected of the best commentators in the industry at the moment, be it as a 'color' commentator, or the one who is more of an analyst and strong in the technicality of the game itself.
You could start off with (not in order of priorities) :
Study / Research about the existing top casters in the industry and learn about their traits and strengths, and if any weaknesses
Learn a lot about your game, especially if you are new, and you aspire to be a pro commentator. A color commentator might seem to be less knowledgeable about the game compared to his analytical counterpart, but there is still a basic expectation of how much a commentator should know about his game. Jargons, technical languages, techniques, skills, etc.
Research on public speaking, as well as how speakers engage their audience in both a formal and informal setting
Learn about the different type of game commentary, which includes both sports and esports.
Learn how to pace yourself, pacing is one of the most important things in speaking in a proper manner that can be easily understood by others. Not too fast, but not too slow either. Try to keep any dead gaps short, and not too much 'uhhs'. It's alright to make a few mistakes at the start, but try to keep the number of times it happens at a minimal amount.
Try to learn vocalization if you can, where YouTube provides a lot of such videos on how to project your voice towards an audience, more than how you would see yourself casting with a microphone all the time. It helps you to deal with a transition from a home setup to a live event.
Practice in front of a mirror thousand of times until you don't get tongue-tied, and you feel that someone can finally understand what you say.
All the best.