Be polite to others at all costs. Actually be a friend to others, even though it is slow, people eventually figure out who the nice people are. Always dress neatly in clothing that fits. Shower and wear deodorant (not meant to insult, but you're 12, and 12 year old kids don't always practice the best hygiene.) Get a good haircut that keeps your hair out of your eyes. Change your clothes every day. (Yes, I know. But it's that 12 thing again. Not meant to be insulting, just realistic.)
Join a sport or take up a musical instrument that interests you and practice it with all your heart. Don't skip any practices. Join a club and volunteer your time to help with it.
Think of what you say before you say it, so that you are not too loud (again, not meant to insult, but you know that some 12 year old kids can be obnoxious. You don't want to be one of them.) Brush your teeth and use dental floss daily. Help others who are being picked on and always be kind to everyone.
Do your best to stay out of trouble in school and out. Don't get into fights. Learn to control your temper. Remember that you can't change what anyone else does, but you can control what you do in response to what someone else says. And that is all that is in your control really. But that's a whole lot!
Study hard in all of your subjects. Get organized and hand in your homework. You don't want to be that kid that the teacher singles out for not having their work. Also, pay attention to the teacher when they give an assignment, so you aren't always the kid who asks, "What page did you say we were on?" like 5 minutes after you were supposed to be working, or when you are called on in class. Read good books and think about them to form good opinions. Share those opinions with others.
Also, think about your goal. Do you want to be the most wanted kid, or just have some of the pressure taken off you? That most wanted kid often has a host of other problems and pressures. That is the kid who is persuaded to use drugs and engage in risky behavior. But you're too smart for that. You know if you are in trouble with drugs, alcohol, tobacco, or risky behavior, that you need to get help from a trusted adult. This puts you way ahead of the game!
As for the crush, you are 12, and it is very rare that the person you like when you are 12 is your life's mate. That is not meant to talk down to you. I just meant that the odds are that she and her boyfriend, statistically speaking, will probably not continue being so close forever. Just keep being nice, kind, polite, saying hello to people, holding doors, smiling, and joining groups of people who are working toward a common goal.
And hang in there.