Do you have a particular style in mind? Here are a couple photos that your description made me think of. What a fun project!

hi meg171. I’m also a teen who loves designing and changing my room. I know your struggle! Hope I can help :)
In this room fairy lights work because of the dark walls, and I can just imagining them as stars.
You could also have a bunch of lanterns, it adds a cool factor to the room.
Here is a cozy room with both lanterns and fairy lights
This looks really almost, magical, so cool.
Fairy lights are really cool to work with, they can hang on just about anything but sometimes they can be a hassle with wires and stuff to plug in, so I would recommend to look for battery powered lights.
If the wood furniture makes the room too dark, you can always paint them a fun color, making it like a focal piece like this one. And of course, find art that inspires you! Art that you really love. I always use Art.com, really good site :)
If you have trouble committing to a bold piece of furniture or certain color you can always have walls and furniture a color that you are comfortable with always, and have accessories, pillows, etc. as fun and outgoing as you like!
I recommend that you have a desk. This is a really cool piece
Hope I could help. Can't wait to see how your room turns out! :)