I’ve answered questions similar to this before so I suggest looking through my blog to see what ideas I’ve sent those people. You’ll also find some good room ideas too!
The first thing I suggest is covering your walls with as many pictures, posters, or even magazine cutouts as you can.
Pictures don’t have to be framed, just use tape! Arrange them in a cute way in order for it to look good. Pictures can be done in blocks since they’re all the same size. You can also hang them on a string hanging from your wall by two thumbtacks. You can do rows of them going down your wall.
As for magazine cutouts, you can get magazines at your local libraries or ask friends for old ones. Cut out images and make collages to post on your wall. I know this is really common for high schoolers. Some people even fill up whole walls.
Another idea is to hang up signs, notes, or drawings that you have made or your friends have made you.
I don’t necessarily recommend doing ALL these things, but I do suggest doing at least two.
To make the room look a little more classic, cut flowers out of garden and decorate your room with them. Tie bunches together with pretty ribbons or rope and tape them to your wall on either side of your window, a bulletin board, picture frames, or shelves.
Make this pretty easy headboard I posted yesterday out of cardboard.
I don’t know if you can afford wallpaper or not, but if you can, it would be great to cover up areas of your wall or a whole wall. You can also cut wallpaper into cool shapes like triangles and arrows and make a cool design on your wall. You can do this with regular paper as well.
As for your bedspread, I suggest buying fabric dye (which is only one or two dollars) and dying your bedspread a solid color. From there you can paint patterns with fabric or acrylic paint and even make stamps and stencils with cardboard to make a cool design. Iron transfers work well too if you can afford it. Hope this helps!