ahhh middle school. that was an interesting time in life! (i feel so old saying that- but i'm 21! It's been awhile since middle school :) )
if i could only go back and talk to my twelve year old self...i'll tell you i guess xD i wouldn't worry too much about popularity. being yourself is the easiest way to be happy. the kids that might say rude things, and hurt your feelings? ignore them! they don't see the amazing person you are :) do the things you love, because if you do the things your friends or other people love, you probably wont be too thrilled about them.
hang on to old friends, but definitely welcome the new ones too! do alll your homework, even the stuff that's hard, or that you hate. Get stuff turned in on time- basically be a better student than I was xD
as far as clothes go, get things you love, and that fit nicely, and you'll look great everyday!