Depends on what type of girl you were and what type of girl you want to transition into. Were you the type of girl who liked wearing just jeans and a t-shirt? Are currently wanting to change into more of a classy, girlie girl?
If you were the type who just wore jeans and a t-shirt, you can update your look by getting a different wardrobe. Such as instead of buying plain t-shirts, buy more girly, cute tops from places like Forever 21. Their style is up to date and affordable! Instead of wearing jeans all the time, try skirts with cute tights.
Also change up your shoes. Don't wear sneakers all the time. Try wearing flats or sometimes wear heels (kitten, stiletto, etc); this includes cute boots! Like over the knee boots-they are super in right now and look sexy with everything!
Don't forget accessories. If you think your outfit is too bland, add a necklace or earrings or bracelets or scarfs or bags. They can really dress up an outfit, but don't over do it! It will look tacky!
Hair: try changing it up with different styles like: braids, ponytails, top/messy bun, straightening it or curling it. If you're brave enough, you can get a new cut or dye it another color or add streaks/highlights.
Last but not least, makeup. Don't ever pack it on and never over do it! Or you will look like a clown! Just use the basics like: concealer, mascara, blush, eyeshadow, lipgloss, and foundation. Makeup can make such a difference!
Good luck everything and remember that practice makes perfect (when it comes to choosing the right outfit, doing your hair and makeup)! :)