I'm a trans dude, so I still get my period. Not sure if that counts, but whatever xD.
I got my period when I was 12, if that helps at all, but then again it's different for everyone so I wouldn't rush it. My friend got hers when she was 7, and my other friend got his when he was 9 (he's also trans. Just wanna clear that up to prevent confusion).
Honestly, just outright tell her, "Hey, mom, I got my period. Can you get me some pads/tampons and give me some tips?", or something like that. She's been through the same thing and she knows what to do. The most important thing is to not panic.
You should also tell your dad. Its best if he knows so he doesn't have to find out and be surprised. You don't want to catch him off guard, because that would be even more awkward.
I reccomend pads if you are just getting your period and tampons later on.
If you have a job, such as babysitting, bring extra pads/tampons with you and change them when necessary. You can also bring wipes and a water bottle. If you get really bad cramps, bring a heating pad (my friend uses a heating pad and it really helps them).
You can expect cramps, fatigue, dizziness, and an overall unhappy/neutral mood. Remember, its different for everyone, so don't rush it :]
- Dino/Wilbur/Toby (he/they/xe)