How do you decorate your room?

Start by taking a very good look at the room as it is now. Measure it, and draw a floor plan to scale. Mark in all of the doors, (closets too) Windows, electrical outlets and switches as well as your TV and phone jacks are. Note the current colours, including the floor coverings. Next make an inventory of the furniture you now have. Okay, now determine what you need in your room, is your bed big enough, do you have a good work area, have you outgrown the toy-box (can it be used for something else). Next you need to know your budget.
Now you need inspiration. This can come from very nearly anywhere. Your wardrobe might be a good place to start for your colour scheme. Magazines will give you ideas of styles and trends. You will give yourself the best ideas (if you are willing to trust your instincts)
Start with what you have to keep, flooring, furniture etc are big ticket items and should only be replaced if absolutely necessary. Check out the local Bed & Bath places, or the Home wears section of a department store and look for something you really love. I recommend comforters that are revisable, (a pattern on one side and a solid colour on the other, works well for versatility. Work from the most restricted colours to the least. Floor coverings have few colour choices; linens, comforters & drapes have a limited number of styles and colours; wallpaper offer a slightly wider selection and finally paint can be mixed to match very nearly everything. Spend your budget wisely, buy good
quality things in styles that are considered "classics" that will last a long time and not go out of style in a couple of years. If you work around this sort of thing you can change the lesser expensive things (like linens and paint) whenever you get tired of them and get a fresh look without breaking the bank. Be cautious of fads, high school musical themed bedrooms won't last forever, they may not last for two years and not everyone wants or can afford to change it all that often. If you are "over the moon" about a fad, buy accessories from it and layer them liberally on your classic room.