When was the last time you saw the floor of your child's room? Is the mess in there piling up to the point you might lose your child in it? If so, let's clean it up. Then let's organize it in a way it will be easier to keep neat.
First, go through all the toys in your child's room. Have two piles. One pile is for stuff to keep. The other pile is for things to give away (charity) or to sell at a garage sale. Honestly, your child doesn't need 100 stuffed animals. Their room is not a big storage space. It's a place to play and sleep. Let them keep any special stuffed animals and the ones they play with. The rest need to go. Also, get rid of any games or toys with broken pieces which can't be fixed, or in reality, won't be fixed. You can make an exception if it's their very favorite toy or game that their grandpa or uncle gave them. But tell them everything can't be their very favorite toy. Go through the books. Keep sentimental ones and ones they might read again. Also, if they have a younger sibling keep books he/she might read. Donate the rest to your local library. Many libraries have monthly book sales to raise money for new books, etc. So not only are you teaching your child how to clean up, but you're also showing them how to help out their community.
Bookshelf Toy Storage
Once the toys and books have been gone through, take a look around to see how to best organize them in the space you have. If there is ample room under the bed, get some rolling plastic bins to go underneath. Organize the toys inside by sizes. If there isn't much room under the bed, buy some plastic bins of assorted sizes. Keep the store receipt and return the bins you don't use. Put games in one bin. Larger toys in another. Toy cars/small dolls in yet another. Building blocks can have their own bin. Etc. Label the bins. Then put them on the closet floor. You can stack smaller ones. If your child doesn't have a closet, stack the bins against a wall. Or better yet before buying bins invest in a bin organizer. You get four to eight metal/plastic bins which fit inside a wooden or medal shelf system.
To save room you can purchase a hanging net to put stuffed animals in. Hang the net in a corner. For the books, hopefully, your child has a small bookcase. If not, consider buying one or making one. For a used one look online on Craig's List. There are some real bargains to be found there. If you prefer to make a small bookshelf you can buy wood pieces and nails at a hardwood store. You can paint and stencil it too. This could be a nice project to do with your child.
Once the toys and books are taken care of, go through any papers. If your child has a desk, get him/her to organize the papers. Afterwards put them inside folders. Put these folders inside the desk. You can purchase a folder organizer. Call your local office supply store to see if they carry them. If your child has trouble getting rid of old schoolwork and artwork tell them they can keep X amount per school year and the rest needs to be released to the recycling bin. Or for the artwork it can be given away to relatives. If your child doesn't have a desk, once again, you can put the keepsake papers in plastic bins. But don't store them in your child's room if he/she doesn't need them. Store them in the garage or attic. Or even the hall closet. The point is you don't want your child to go through the papers again and then have them strewn about the room once more.
Hopefully by now things look neater. If the room still seems cluttered, look around and ask yourself, "Does my child need every item in here?" Remove excess furniture and large toys your child doesn't use. Maybe it's the rocking chair you rocked her in as a baby. If she doesn't use it, remove it. She can use the space. If there are any big toys, such as a plastic doll house that hasn't been used in ages or a frog bean bag chair that has a leak, either put it in the attic or get rid of it.
I saved organizing clothes for last. Pick a period in the day you and your child will have enough time to go through their clothes. Go through all their drawers, as well as the closet, to see what fits and doesn't fit anymore. Clothes that don't fit, but are still in good use, can be passed down to a younger sibling, a relative, or a friend. Or you can give them to a homeless shelter. Once you have gone through their clothes, teach your child to fold their clothes neatly. You can label what goes inside what drawers with removable sticky tape if you want. It's easiest for children to have separate drawers for each clothing type. A pants drawer. A shirt drawer. A PJ drawer. A sock drawer. An underwear drawer. Though the socks and underwear can go together to save space. If you live where you have seasons, you can save space by using the drawers for only seasonal clothes. The rest of the clothes can be stored in bins and taken in when that season arrives.
If you really want to get into organizing you can organize by clothes and color. Take the closet: All plain shirts are hung first. Start with the color white and go darker. White. Yellow. Light blue. Violet. Dark blue. Brown. Black. Next, hang the fancy shirts. Go from white to dark again. After that, hang the dresses. Then the sweaters. Then the jackets. All by light to dark. This looks great, but remember your kid is just a kid. If it doesn't stay this way, let it go. With a messy child just be thankful if they can keep their room clean for a week.