Teenagers can be frustrating...we nag them constantly to clean their rooms and they ignore us, but when they decide for themselves that they want to do it, they want us to help them.
As frustrating as that may be, it is actually a good thing that your teenager is now showing an interest in cleaning and organizing her living space and I would recommend that you help her as much as possible. One word of caution, though...be prepared for the probability that after all of your hard work, the room will not stay as neat and organized as you get it...and that's okay. You are still helping your daughter learn a wonderful life skill that she will eventually benefit from.
The first thing that I would do is completely clear out the room. When the room is empty, give it a thorough cleaning from top to bottom. You want to create a blank slate.
Next, get a trash bag and some boxes. Use the boxes for items that your daughter no longer wants to keep. You can either place these items in a garage sale or donate them.
As your daughter to analyze each item before it is brought back into the room. She needs to decide whether she wants to keep it or get rid of it. As much as possible, let her make these decisions on her own.
The rule is....every item that comes back into the room must be assigned a designated home. If your daughter knows where everything belongs, she will know where to put it away when she cleans her room in the future.