heavily personalized!! a lot about emotions and expressing yourself through decorations; polaroids and photographs, vinyls, posters...
darker colours for walls are common (reds, blacks) but white is a good colour as well, especially if you're going to cover it up with posters and other things
fairy lights around bed posts, mirrors, or certain parts of your room can highlight a main piece. if you have one wall dedicated to art/posters/pictures, putting up a string of lights as a border can draw attention to it. it's also cool to get tinted lights to fit the theme of your room. neon light signs can also apply here!
a really cool thing i've seen is that people use empty alcohol bottles as flower vases. it's edgy, but pretty at the same time, and is a really good decoration piece.
some things to hang up around your room: band/music posters, photos or polaroids of things like landscapes, friends, family, etc..., vinyl covers (and even the record too), art, quotes, wall tapestries or flags