The first thing i would do is take a garbage bag and start removing most of the items from her room.
I would leave her about three changes for school and one for the week end.
Shoes, i would leave her sneakers, and one pair of dress shoes.
All those posters on the floor would be boxed up, as well as any thing else that was not essential to her that was on the floor. Everything, that was not being used and was strewn on the floor.
All school books and papers would be boxed up, were she could get at them if she really needed them.
After I had done this, i would firmly tell her keep it clean, or not, but know this if you can not show enough responsability to keep it decent, then there will be nothing added to your room, like new clothes, make up, posters, ect simply because you can not handle the reponsabilty of managing items is a mature and responsible way.
If after a week she is showing that she is making a sincere attempt at keeping her room decent, let her go through her clothes and pick on more outfit, to add to her closet, keep doing that till she has earned her clothes, make up, posters, what ever back. If she starts slacking take an out fit away, poster's away, make up away, ect. Do not buy her anything new untill she really gets that you mean buisness. I mean really gets it that you had enough and are through playing games. When you feel she really has started to get it and is sincer about making a concious effort to do better reward her, take her out to a movie, or let her have a sleep over ect. Honestly if my house was in such a mess i would not want people over, so she should nto be having people over into her space (bedroom) untill she got it under control.
Be aware she will probably react like any typical teen, temper tantrum, crying, begging, promising, ect, don't buy that for a second lol. Do not give in, she is fourteen she really needs to learn consequences for action, respect for her property and yours, and self pride in doing a job well for herself.
This is a learning process, so it is not going to happen over night, it is a lot faster to learn a bad habit than it is to undo one, so patience and persaverance is key.
Ignoring a problem does not work, it simply reinforces for your daughter, that it if you do not mind it does not matter, and believe me it really does matter. There is a lot of teaching opportunities that are missed when something is ignored, and she is going to need all the tools you can give her to make a wonderful life for herself when she reaches adulthood.
Good luck! Let us know what you decided to do, and how it is working for ya, or if you just need some support.
Tammy