Be well-groomed. This is essential if you are even going to approach an attractive girl. Brush your hair, shower, wash your face, brush your teeth, use deodorant and maybe even a little cologne, and apply acne medicine daily (if you have a problem with your skin). Don't wear your pants off your butt, often called "sagging": it's not that attractive to the majority of girls. Wear clean, attractive clothes that fit. (If in doubt, get an older sister or close female friend to help you in this department.) Don't wear skin tight clothes, but don't wear clothes that are too baggy either. Girls hate that.
Have a great attitude. Be fun to hang around with, easy to laugh with, and be outgoing. Just don't be full of yourself. A girl may like an overly cocky guy at first, but eventually, she will find it selfish. Don't try to impress everyone: girls like humble guys. At the same time, a sense of humor is always good. Girls love when guys can make them laugh.
Show respect. If you respect everyone and give them their space, they'll all respect you. With girls, it works even better. If they see you respecting everyone, and not getting pushed around while you're at it, they will be reassured that you will treat them right. Be kind to all.
Have nice conversations. Try to aim for you both to be speaking half of the time, but make sure that you are saying something worthwhile. It's not attractive when only one person talks endlessly or when they say bad words. Another thing to watch for is her attention. If she isn't paying complete attention, do not repeat yourself several times. This makes girls feel you are desperate, and usually annoying.
Show interest in her. Let her talk. The #1 mistake when talking to a girl is focusing on yourself. Girls are way more comfortable when they talk about common interests. Ask about her interests, hobbies, favorite books, music, etc. If she asks you a question, answer it in a few, short sentences and then redirect the question back at her. This engages her in the conversation. When in doubt, compliment her like your hair looks nice today. Even if a girl does not like you yet, complimenting her will make her feel good about herself and think positively about you. However, if she keeps redirecting the conversation back to you, talk about yourself, otherwise it seems like you don't really want to be talking to her.
Try flirting! Don't be obsessive! If you both make eye contact, do not be the one to look away, but do not stare for too long. When you both make eye contact, just give a slight smile. Touch her arm or shoulder for a few seconds. Physical contact is a great way to show you're interested. If she seems uncomfortable, do not do it again.
In general, you should leave physical contact until you both get to know each other more, especially if she's shy. Touches, even to areas that aren't considered taboo (the arms, hands, shoulders, lower legs) may be interpreted as sexual cues if done at too early a stage in the relationship. She may think you're just trying to have a casual sexual relationship, which will usually scare away most girls. Girls will generally break the touch barrier themselves when they're ready.
She may look away and she might blush a little. Be careful though; you don't want to stare her down. A good move is to look away, and if she likes you she will shoot you glances. Watch for that. And be courteous. Open doors. Do kind things, girls love that.
Respect her friends and their ideas and opinions. Always speak highly of her and never talk trash about her. It will damage her reputation and yours. Hopefully then it will work both ways - she will be tolerant of your friends. Be careful not to compliment her friends too much, however. Example: You may say "Your friends seem cool. We should hang out with them and my friends some time, I think they'd get along". You may not say "Sally is so hot. She has the nicest eyes". You may think getting her jealous is a good idea, but it just makes them angry and hurt. Doing this could also sow discord in her friendships, you may actually create bad feelings between her and her friend(s), so make it very clear that you're interested in her only, and that you're merely acquaintances with her friends.
Have your friends be nice to her It's a fantastic thing when your friends don't make a total fool of you. Warn them ahead of time and avoid topics pertaining to embarrassing moments of your past or awkward jokes that make very little sense. Don't introduce her to your friends right away though. You want her to like you, not start dating one of your buddies. If your friends like to embarrass you, try to avoid them altogether.
Be polite to her parents. Be kind to everyone, especially them. Don't be too over-the-top charming with the parents, however, or they'll think that you have something to hide.Be willing make sure that you and her father get along as he will be looking out for his daughter.
Don't criticize her. Unless she asks for constructive criticism. Even in these cases, she probably wants you to compliment her.
Be romantic, but not over the top. If you've been dating for a while you may attempt a grand romantic gesture, otherwise, you'll look like a stalker. It's also okay to be a little cheesy, like giving her a bouquet of roses and a loving poem. It may sound like a bad idea, but girls will find it thoughtful if it came from your heart. A better idea than being cheesy is to really LISTEN to her and remember things she likes. Then take her somewhere she's mentioned wanting to go, or buy her the books she's been wanting and hide a little note inside. Flowers and candy are really cliche and don't really mean much since anyone can just buy someone flowers. Showing you've been listening to her wants and needs goes a LOT farther than roses!!
Start talking to her casually. If you don't know her, make friendly conversation. Ask for the time, and/or compliment her watch. Avoid throwing too many compliments about clothes or shoes around: that might make women think that you are homosexual as many gay men are fashion-forward.