I would like to second what Nilaman said. To reinforce that statement, you would need training to effectively use a knife in a self-defense situation and I rather doubt that you will find anyone willing to rain a 12-year old in such martial arts. If you are in a situation where you are being threatened, try not to ever be alone, try to carry something like a length of broomstick with the ends rounded so as not to be sharp or jagged, or a knee-length sock loaded with sand or even just damp dirt, hard packed. Any of these will provide better protection than a knife and be less likely to get you hurt. They are also, on the whole, less likely to land you in trouble with the law. Well, the sock might in some jurisdictions where it could be seen as a form of club.
