Wall decals and pretty curtains and bedding sets will amp up any room.
I would just scroll around on Pinterest or DIY sites, to find interesting things. Not only can you find things to add to your room, but you can spice up anything you own. I'd just google "DIY ____" depending on what you plan to add/change about your room.
Personally, as a nature kind of person, my room is filled with random things I find. I keep old bottles, and use them to hold peacock feathers or fill them with seashells. Live plants and large crystals (think geodes or clusters) look beautiful ; also, the plants filter the air, and there's all sorts of spiritual beliefs about crystals and their benefits. Indoor plants can be found at Home depot or Lowes, while crystals are rather harder to find (online stores, gem shows..etc are good places). If you have any nature trails, pieces of driftwood, feathers, and pretty stones can be found there.
The reason why I like the nature idea, is because not only is it a reflection of what I enjoy, but they also have stories behind them and how I found them. Try to do this if you want to add meaning to your decor
Many home improvement stores have cheap things that could spice up any room. Try going to Ikea or some other home decor store and see what you can find that interests you.
How do I spice up my room? What can i put on my dresser and nighstand besides my lamp? And my walls are so plain! I cant find any paintings, i tried. HELP!