HAVE YOUR OWN LIFE: When you are trying to get in a relationship or make a girl like you and you have a boring life, you are making the packaging look great, but in reality... you have no life.
GOOD HYGIENE: Go take a shower, rub body soap all over you. Twice. Shampoo your hair, dry yourself off, apply fragrance, put clean clothes on, and you're set.
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ASK HER TO DO THINGS: You ask a girl to do something for you, she does it, she subconsciously likes you a bit more. "why though?" Well, when we do stuff for people, our brain thinks since we do something for someone, we must like them.
MAKE HER LAUGH: It proves that you're intellectually strong and humorous, which is a desirable trait in a future mate. Also, If her eyes dart at you first when she laughs, she probably likes you at a subconscious level.
SHOW, DON'T TELL: If you really want to get a girl to like you, you need to invest in her. This means with your time, your money, and your emotions. Don't just say if can't do for her.
FAMILIARITY EFFECT: girls like guys who are more familiar. This is why so many couples end up getting back together 10-20 years after high school, because they're familiar with each other.
BEEF UP CONFIDENCE: The best general advice I can give you to become more confident is to workout regularly. If you're in middle or high school, I suggest weightlifting 3 days a week: arms, legs, and core or some cardio.