Your daughter needs a place where she can relax and be herself. A place for alone, personal time, and also an awesome hang out space when friends come over.
Just like any redesign or staging project, start by purging! Help her go through all her stuff and decide what stays and what can go. Special things she has outgrown but you want to keep, can be placed in “memory boxes” and put in the attic or basement. This goes for all the clothes in her closet too.
The junk she doesn’t want can be donated, recycled, or brought to the trash bin.
Have her decide on a color scheme or theme that is personal and means something to her.
Walls. Paint one wall or wallpaper with this favorite color. If it is bright,,keep the rest neutral with white, cream, grays and browns. Add art, shelving, a cork board, and a full length mirror.
Soft goods. Be sure to have soft bedding, lots of extra pillows, throws, and cloth window coverings to add to the cozy.
Seating. A spot for the friends: bean bag, moon chair, ottomans, or even a small loveseat.
Closet. Use it to full capacity, but do it in an organized way with bins and shelving.
Create a spot to do homework with good lighting.
Get extra lighting. This can be done by stringing lights, adding an extra lamp for reading, etc.