It's a well-kept secret that every woman at some time in her life has had to ask another woman for a pad. Yes, it's embarrassing, but you can rely on other women to see you through this situation. And in all honesty, first periods were designed really well because it is usually so little that it won't seep through to your outer clothes. Everyone is different. It can last between two and seven days.
Most girls have it for about five days.
In addition to what's covered above, it's a good idea to have feminine products ready for that first period, and to know how to use them. If you have a pad with you, go to the girls' room and put it on. If you don't have one, go to the school nurse, the office, or wherever you can get a pad. If you don't need it, maybe one of your friends will and she can return the favor some day soon. It's probably just a heavy flow, which can happen during a period's first day or two.