Every writer has a voice. It's what makes your writing uniquely yours. Some writers are formal and academic. Others are casual and conversational. Neither is wrong - they're just different.
Finding your voice takes time. You have to write a lot and figure out what feels natural to you. Don't try to sound like someone else. Be yourself.
Voice Comes from Authenticity
The most engaging writing comes from authenticity. When you write what you genuinely think and feel, your voice shines through. When you try to sound like someone else, it rings false.
Think about how you talk to your friends. That's probably closer to your natural voice than how you'd write a school essay. Bring that conversational quality into your writing.
Practice Makes Perfect
The more you write, the more your voice develops. Don't force it. Just keep writing, and eventually, you'll find yourself. It happens naturally.