First if you are not grown enough, do not talk to strangers. Stick to well lit public places and make it a habit to walk around with a friend or relation. Be alert and aware of your surroundings and avoid those spots that are known to be not so secure. If you suspect you're being followed, head straight into the nearest store or restaurant. You can also take self defense classes, just to feel secure.
Keep always your cell with you, and the emergency numbers, try to avoid to move alone, be always with some elderly person, always be alert, try to be in contact with your family members when you are alone, don't talk to strangers, do not accept any thing from strangers. In the evening time you must be at your home until and unless it is an emergency. Always let your parents the exact place where you are and try to get some basic knowledge of martial art and can keep pepperer gun.
Don't be skinny. Chubby people are harder to kidnap. That's why I always carry a couple extra pounds. If ever faced with a situation that you're about to be kidnapped, yell fire. Fire will get people's attention a lot better than help.