I agree with Snail, Fishcakes. My mum kept a pack of pads handy specifically for the day when my sister and I started our periods! Your mum must be expecting it, and she will be excited that her baby is growing up. Even if she's not well at the moment, you're not exactly going to go pounding round the house ringing bells and screeching "I've started my periods!!!!" (please tell me you're not...)
Just say "mum, have you got any pads, I've just started my periods!"
(says pee, having muttered and mumbled and blushed when she told her own mum...
). If you want any advice from her, make sure she realises that, because she might think that all girls learn everything about everything at school these days. I'm sure she'll be pleased and flattered to answer any questions you might have.
Shine your light and let the whole world see.