[edit] Steps

Smile. People who smile are generally happier, healthier, and have more friends!
Think of your good qualities; you have a lot! Go in front of the mirror and say something nice about yourself. Believe in yourself so that when you go out there, people admire your confidence.
Look good. Care about your appearance and about your clothes. They don't have to be expensive, they just have to look nice. Have your own unique style; don't try to dress like anyone else. Take care of your hair, scent, body, skin, nails, and teeth.
Compliment people you meet (if you see anything you like.) Meet lots of new people and be friendly with them.
Be flirty sometimes. It is fun to smile at, look at, and talk to your crush or any cute guys/girls that you like.
Throw parties when you can and invite people from your classes. Don't let it get too out of hand, though, or you could be in big trouble.
Pay attention to classes, study, and complete homework. School is important, no matter what other people say.
Be yourself all the time, because pretending or imitating someone else is not good and not comfortable. See your awesome qualities, improve your bad ones, and be proud of who YOU are.
Don't stick to one person all the time. You can have close friends, and even a best friend, but if you stick to one friend at all times, you'll never meet anyone new.
Be up to have fun. If anyone suggests something to do, as long as it's legal, healthy, and doesn't go against your principles, don't turn it down. Be known to be someone who will support anything fun. It is exciting and it makes you grow when you jump out of your comfort zone. But don't do anything that feels uncomfortable for the sake of being cool!
Make friends with energetic, fun-loving people. This will ensure that you have lots of fun all the time. But this does not mean that you should ignore quieter people or not be their friend; try to make friends with everyone. Just make sure that at least a few of them are fun for you to be with.
Always have something up. Every week or so, start an new fun project such as writing a song, and let some cool friends know about this. Always be looking forward to something--a school play you're going to be in, a party, a concert, or a carnival.
Be known on the social scene. This means you go to all the cool parties, concerts, join school clubs, go out, and be known for your talents. Especially work on your natural talents; this will make you feel confident and happy. If you're good at dancing, join a dance club and perform at talent shows.
Look friendly and approachable. Laugh, play and act silly. This shows people that you're confident and fun and people will want to get to know you.
Make sure you're in on all things cool. Learn the new popular dance, popular music, and get to know popular people. Don't get sucked into the "I have to be just like the popular kids" mode, though. Keep your own style and personality alive.
Say what you want. Even if it's "uncool". You need to stand up for yourself and your beliefs. Be the change you want to see in your life. However, do not say offensive or hurtful things to other people. Not cool.
Be random. Say unusual things. Stand out from the crowd and keep it interesting. Try not to be too unusual, however, as this may backfire and make you seem unapproachable.
Laugh at jokes that are funny, but don't feel like you have to laugh at jokes that aren't funny, if people ask you why they you aren't laughing at their "funny joke" then say why you think its not funny.
Dress in a way that you like, makes you feel pretty, and isn't an eyesore for others. Don't dress in a inappropriate way that only gives people the impression that you are asking for attention.
Be modestly sarcastic. Don't always say mean things, but if you see the irony in something, people will appreciate your wit.
Be smart and act smart. That doesn't mean you should be a nerd or correct people whenever they make mistakes. Just make the best choices possible and make your ideas and work intelligent.
Don't do things just for the sake of looking or feeling cool. That's the most uncool thing that you can possibly do, and later you will look back on what you did and say, "Wow, look how dumb I was. I thought I was so cool, but I was just being stupid."
Like who you are. After all, if you don't like you, how can other people? Be comfortable in your own skin, and make choices that will allow you to respect yourself. Respect for yourself is ten times more important than others' respect for you. YOU have to live with you, not anyone else.
Don't get in fights with people. It never works out and only causes hurt feelings. Fighting just makes more enemies, and that is not what you want.
Learn the above rules, and then forget them and don't worry about "being cool." The coolest kids are the ones who don't care too much about looking cool, and just do their own thing, their own way (within reason).

[edit] Tips

Self confidence is a virtue, if you care too much about what others think about you, you will never be cool, just annoying and not fun.
You don't have to be shy, if you want to meet someone, go ahead and introduce yourself nicely. At parties and prom, be confident and go up to the person and be polite, smile and ask this person what you want. You don't have to be shy, most people like an outgoing person.
Everyday when you wake up, breathe deeply and imagine that you're aspirating positive things and avoiding things that are bad for your personality. It'll make you feel better, like you're more pure inside and it will just make you relax. Seriously, it sounds weird, but it does work.
Be very nice to everyone, but if someone bothers you, forget it and defend yourself saying what you think, NEVER let anyone bring you down.
Never forget who you are. Being cool is simply a trait you have that will be seen by others, not a personality change. Don't change yourself drastically just to be cool. Stay true to yourself so people can like you for who you are.
If your about to do something to draw attention to yourself, put yourself in onlookers shoes, EXAMPLE: You feel like flicking an eraser at somebody sitting across from you in class, just for fun, would you want to have an eraser flicked at you at that time, and how would you react in the situation? don't put them in an uncomfortable spot.
Don't change yourself too much!
Have fun! If you are considered cool, but don't enjoy it for various reasons, what's the point?

[edit] Warnings

When you become cool you don't have to ditch old friends because they're not as popular as you; real friends are so much more important than being a cool kid.
People may not suddenly start thinking you're cool, or even if they notice, they may not be rushing to be friends with you.
Being cool doesn't mean get addicted to drugs or smoke.