ANDY'S ANSWER: I'm sorry that your period has caught you by surprise, but it's a natural thing to happen when a girl starts maturing. It's not too hard to be prepared. Buy a box of pantiliners and another box of heavier pads. As soon as you see some sign of a period, (red or brown marks on your underwear) you can start wearing a pantiliner to protect them. As your periods get heavier, you might need to switch to a heavier pad.
Later, you might even want to try using tampons, but I honestly would not recommend them at first. Pads are easy to deal with and it allows you to see how heavy your flow is. Until you become regular, it's probably all you'll need to use.
Below is the recording of Andy answering this question.