I started mine when I was 10 yrs old and I was very shy about it! At the time I considered myself lucky that my mom seemed to be, too... or maybe she just didn't know how to talk to me about it or didn't want me to be embarrassed. So, she bought me a starter kit before I ever started. It came with a video and a bunch of reading material that answered all kinds of questions (some I had in mind but was too afraid to ask & some I hadn't even considered).
I was embarrassed by the subject no matter how it came up, even if it was discreet & brief! So, I can understand not knowing what to say! But, it's scary to think that you could start at any time and not be prepared! Then what?? Don't worry, though. ;) We girls have all been through it, we all understand and look out for eachother! So, even if you started and weren't prepared, there will be other girls/ladies around that can slip a pad your way (without embarrassing you) when the time comes. ;)
Everyone's body is different; some bleed heavier than others and some bleed longer than others. Some can almost do a countdown (like the space shuttle) to the time they'll start their period while others never know (I never knew when I would start mine until it regulated a few years later). You'll figure that out with time.
If you're like me and want to prepare yourself by having the essentials with you when the time comes (and I completely understand! I still do this), don't be afraid to ask your mom or any other woman you trust and feel comfortable with (a teacher, friend of your parents, etc... although your best bet is mom, because she can keep them coming when you need them).
As far as what to say, there isn't really any right or wrong way to ask for help; just ask whatever way you feel comfortable asking. I would suggest asking her for one of the 1st period kits (you can find them online) like I got and just tell her you haven't started yet, but know that it will happene eventually & you don't want to be caught off-guard. Trust me, you don't really need to explain anymore than that - she knows!! Maybe try asking her when you're in the store near that section; that way you won't have to bring it back up to remind her, later... and since she's on a mission checking off her shopping list, the attention is drawn away from it, right away. ;)
It's awesome that you're asking, now, instead of waiting for it to happen, as if it won't... because then you wouldn't be prepared and it would be a rough day. Just remember, we've all been there or will be going through it, too... and even though it's embarrassing to talk about it, now, one day you'll be giggling about it with us. ;)