I can't tell you exactly when it'll come, but I can tell you some common signs that will suggest it's on its way.
First, the normal puberty stuff - hair under your arms and on your genitals, your breasts growing, etc.
In addition, a particular sign that your period will be coming fairly soon - usually within a year - is discharge. This is what you would see on your underwear. It's usually clear or white, and should not smell funny or be an unusual colour. This is how the vagina cleans itself.
If you feel concerned that you have not started your period yet, talking to a doctor might be able to make you feel better. If that makes you nervous and you've got an older female relative, that could help. Many girls will get their periods around the same age their moms did - so that might be worth asking.
In the meantime, you don't have anything to medically worry about. It's okay to be 13 and not have your period yet; based on your age alone, I would guess you'd be getting it by the time you are 14 or so. If it doesn't come, a doctor will be able to check things out and make sure everything's okay.
Best of luck.