If you don't read, you can't write. Do your homework, i.e., master the mechanics of grammar, of language. Listen to them, the way they talk, what they say as well as what they don't. Watch their body language, the way they move, how they interact with others. These are the models for the characters you'll invent.
People are multi-layered.
Your characters had better be as well. Be prepared to face rejection, isolation, frustration and constant bouts with a lack of self-confidence. Keep writing anyway. In this instance, you learn by doing. If at all possible, link up with a critique group.