Figuring out the subtleties of mic placement and styles takes a process of trial and error. In public speaking it is more about figuring out the techniques needed to project your message. When placed too close mics distort your voice. Sometimes they make you sound too loud and sometimes they're too soft. Using a few simple tricks and techniques, anyone can sound like a professional public speaker on a microphone.
When speaking into a microphone, remember that all it can do is make you louder. It can not add confidence to a weak voice, or turn a whisper into a yell. Project through the microphone and speak from the chest, but don't yell.
Be prepared to hate the sound of your own voice, if using a mic is new to you. When you speak your own voice sounds different to you because it reverberates through the skull before reaching your ears. You associate that reverberation with the voice you hear in your head, the one you think or read with. Realize you are the only one who notices this difference. Become used to the sound of your own voice so you can speak with confidence.
Understand the most basic pick-up patterns. All mics have a certain area around them where they pick up sound the best. This area is called a pick-up pattern. Some mics are super directional (meaning they pick up sound mostly from exactly one place) while others are omni-directional (meaning they pick up sound from everywhere around them). Most mics fall in-between these two extremes, so it's a safe assumption that any mic will pick up voices better if they are directly in front of the cylindrical part.
Speaking on a mic is a skill. The more you do it, the better you sound. The best way to practice is to stop self-editing in the moment. Instead, review afterwards how to do better next time.