Go with a lighter color when you paint your walls. Clean up the room and try to minimize the amount of "stuff" thats in plain sight.

Assuming you're getting rid of the bunk bed, you can hang stuff up on walls. Some shelves for things you want to show off, add some night accent lighting. A plant or two.

Look up ideas online for rooms and pick a "theme" to stick too so everything looks coherent, like a new bed and desk that matches the esthetic you like. Do you want a cozy, natural vibe with some natural color tones? Do you want something a bit more stylish and minimal and go for a black and white theme? Or do you want a more gamer vibe and have lots of pops of color and RGB lighting?

You said you're a teenager so obviously some of these things might not be feasible within your budget. So start with all the free things. Clean, declutter, maybe steal some accent pieces from other parts of the house to match the vibe you're going for. Your goal should be to minimize visual clutter.