Periods are a natural, healthy thing so you shouldn’t be afraid of talking about them to whoever you want, whenever you want!
Mums and older sisters are great to talk to, because they’ve been through it and can offer advice. Friends are a good support, too - especially if you’ve run out of pads and tampons.
Periods shouldn’t feel like a taboo subject, so be open with other family members. Dad probably understands more about periods than you think!