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This is the most important thing. If you have had your period for a while you will know which products you prefer, how heavy your flow is and if you prefer tampons over pads. When you have just started most girls use pads. Even if you don't have problems with irregularity, a friend may be in need for supplies. It's always better to be prepared. Ask about which products you should use, or stick to the regular ones for your age group. Order free samples so you can try them all (don't try them at school or work because it would be better to leak at home than in public.) Don't forget that she understands. After all, you are both women.

Summary:
Have enough supplies. Always keep spare pads and tampons with you. Talk to a trusted adult, best friend or sibling. Realize that it can be embarrassing to talk to your mother about these things.