Just because you're 13 and your period hasn't started yet, it doesn't mean there's anything wrong with you. Every girl is different, so there is no "right" or "wrong" time to start your period. Don't stress about it!
Most girls usually get their first periods between the ages of 10 and 16, which is about 2 to 2-½ years after their first signs of puberty (breast development is usually the first sign of puberty, but sometimes pubic hair comes first). However, some girls start puberty a little earlier or later than others, which is still normal. Some girls get their first period as early as age 8, and some other girls don't get it until they're almost 17 or 18. There's nothing wrong with that.
A clue you can use to figure out when you might get your period is to check your underwear for vaginal discharge. This clear or whitish, mucus-like discharge usually appears about 6-12 months before you get your first period.
You can also ask your mom how old she was when she got her first period. Girls who get their first period later than average often find that their moms or other female relatives also went through puberty later than average.
Just relax -- it'll come when your body is ready for it.
For now, you should enjoy not having it. It's a pain -- literally! Get ready for cramps, bloating, headaches, acne, irritability, constipation, etc., etc... the list goes on... ;)