1. Change your tennis shoes and wear flats.
2. Get a nice hair cut, with flippy layers, or wear a ponytail with a few strands of loose hair.
3. Get a pretty smelling perfume.
4. Blush and lip gloss! They are HUGE if you want to look girly.
5. Paint your nails a pretty color like pink. If you REALLY want to look girly, make it glittery, or if pink isn’t your thing, get a french manicure.
6. You will come off a lot more girly if you smile and laugh a lot. It makes you look more carefree and innocent.

I hope this helped!
References :

Edit:
Most girly girls at my school wear things from Aeropostale and Hollister (although, around here at least, Hollister’s shirts are more popular with the gys). Skinny jeans are very "in", as well as converse, but so as to not give it a "tom boy look" you can it with a bright top, put your hair up and something that really stands out from what everyone else is wearing (what’s in style).

As for makeup, try a natural look (make people think you were born like that ). If you want some to see a little makeup, you can, just don’t overdue it with neon bright colors.

Examples:
Clothes:
Dark blue/black skinny jeans, converse, hot pink top, hair in a sleek bun, and big silver hoop earrings.

Makeup:
Black eyeliner on just the top lids, concealer, lotion, light eyeshadow, lip gloss, mascara, and blush if you want. All that match your skintone.

Hair:
Up, to show confidence. Buns, ponytails, or something that can look casual but is really different.
References :
I can dress like this I just choose not to :p