How to tell my brother i started my period?

I'm sure this won't be any huge surprise to your brother since he sounds like an adult with a job. Just tell him your period started and you need some supplies. Probably the school nurse told you what you'll need. Really, it's no big deal. Just tell him in a casual manner that you'd use to tell him you needed more shampoo. No biggie. I'm sure it will go just fine.