Best to keep a diary of your menstrual cycle so that you know roughly when to expect the next period. It is always a good idea to always have a sanitary pad or panti liner in a secret compartment of your bag or purse in case of your period starting a little sooner than expected.
I told all my friends but I got mine when I was 11 so I started telling when I was about 12 and they started coming To me for advice and I could go to them and it's good to know so then you can borrow tampons or pass if you have to
I never told my friends, and my friends never told me. It's something we all just assumed about each other. Only time it ever really came up at all was if someone forgot to make sure they always had a tampon or pad in their purse just in case and needed one.
But yeah, it's up to you. If you want to tell them, then tell them.
It is up to you. You are not obliged to tell your friends anything.