I think spending many hours browsing a wide range of stores and their clearance section will make your dorm the most personal it can be to your liking. Shops like Target, Hobby Lobby, and Walmart can be your best friends for decorating your room on a budget. Also try local thrift stores for statement pieces for cheap, like posters, mirrors, frames, etc. I would recommend buying plants, some type of decorative lighting (a decorative lamp, a night light, string lights), and posters for your favorite movies, shows, or bands.
There are so many cheap places to buy dorm decorations! My favorite place to shop is Goodwill- I shop there for clothes, shoes, accessories, and decor! You can really find some gems in there, I found a $300+ Kitchen Aid mixer for $80 brand new! Also, places like Big Lots, Walmart and Target are great too!
Goodwill is the best! They have so many good decorations that you can get creative with, and it's super cheap! Otherwise, Target is great as well.
When I was in a dorm room, I used a lot of pictures, tapestries, and canvas prints to decorate (and I still use these items). Printing pictures is pretty cheap, and it's nice to have that taste of home in your dorm room, and you can find tapestries and canvas prints for decent prices as well. I did a lot of my shopping on Etsy, Target, Dormify, Society6, and Amazon. You want your dorm room to be a sanctuary from your classes and school life, so you want to make it as homy as possible!
I mostly got decor from Target and WalMart since they were affordable and convenient. I didn't go too overboard with decor but I did hang up photos that I'd gotten developed of friends, family, pets, scenery, etc., to make my part of the room seem more home-y to me. It really depends on your personal style and likes as well as how much empty wall space you have to decorate!