[edit] Steps

Don't be loud and obnoxious. This is a major turn-off and people might think that you are annoying. Be friendly and happy to others and have good manners. Say please, thank you, and try not to swear a lot. Be nice to your family and friends. If someone is being mean, just ignore them.
Be a good student and pay attention in class. Try to get most of your homework done and get good grades. Not only will your parents be proud of you, it will be very helpful in the future.
Be clean. Nobody likes someone who doesn't bathe or smells funny. Make sure your wash regularly, brush your teeth, wear deodorant and style your hair nicely. You don't have to be gorgeous, just look presentable. Being organized helps too.
Dress modestly. Wear nice tops and sweaters, jeans, not too revealing skirts and nice dresses. You can create your own style and mix in your personality also with cute jewelry and purses. If you like makeup, don't go overboard. A little mascara and eyeliner, and maybe some lip gloss. Keep it natural and simple.
Have hobbies and like cute/girly things. Drawing, painting, reading, dancing, ballet, ice skating, sewing, animals, horseback riding, girl scouts, playing instruments, cooking. Anything that you think is fun and enjoy doing!
Stay away from bad things like drugs and smoking. These are not only dangerous, they will also give you a bad reputation and you could get in a lot of trouble from your school or your parents. If you are allowed to date, date nice boys and keep things simple and innocent. Small kisses and holding hands are okay.