My DD also started her periods at age 10.
I assume she already understands what a period is, and why it happens? Even if she does, you may want to reiterate. I'd focus on checking she is alright - letting her know that it is normal to have cramps, to feel tired etc. But that every woman is different. I gave DD a few pain killers so she could take them when she needed them without asking, but didn't have complete free access iyswim. I gave her Calpol melts at first (easier for her to control). You might also want to try a few different pads - DD found Lilletts teen range best as everything else seemed too big. I did also talk about other options, but she was a bit overwhelmed and didn't want to consider anything else at first, which is of course fine.
Is she at school this week? If she's not, that makes things a bit simpler. In terms of school, DD found it helpful to talk to a few trusted friends - even though she was the first in her year group to start her periods - it was nice to get their support. She used to take a pencil case containing pads, painkillers and spare knickers (just in case) to school. I'm sure it is frowned on to let a 10 year old have painkillers, but there was no way DD would have asked a teacher. Is there a sanitary bin in the toilets she uses? Some schools are woeful in this respect, so you may have to advocate on her behalf for her to use a separate toilet.
The plus point for DD, although she clearly didn't appreciate it at 10 is that, as the first person in her peer group to start her periods, she has become the "go to period expert" among the other girls. She quite relishes this role :)
I assume you've also had chats about privacy in your house? I'm not sure it's appropriate for your DS even to be going in the bathroom if DD is having a shower.