1. Bedding: Chances are, your bed is going to be the largest piece of furniture, and since you won't be able to paint, I'd recommend getting a bright comforter or duvet cover as an easy way to add a punch of color or pattern to your space. And since you're stuck with a Twin XL, don't hesitate to avail yourself of all the colorful kids' bedding out there. Just because it's marketed for children doesn't mean that it's immature. Some of my favorites are this otomi-patterned duvet and Faculty Mixer bedding from Land of Nod, the orange geometric comforter from Target, and the Frankie duvet from Serena & Lily.
If you can't find anything you like that's suitable for a Twin XL, you can always move up to full-size bedding. The fitted sheet can sometimes cause problems though, so here are some good Twin XL sheet options: herringbone sheet set, urban ikat organic sheet set, Oxford stripe linens, and 400-thread count Egyptian cotton sheet set.
And finally, I suggest outfitting your bed with more pillows than you normally would. If you have your bed against a wall, you can use all those extra pillows to line the back, making it more like a sofa for when your friends come to hang out. Check Etsy for some great accent pillows, or make your own using punchy fabric and this no-sew pillow method.
2. Rug: Cold linoleum can verge on "institutional" really fast, so adding a rug is a quick way to really soften up your space and give it a more comfortable feel. Even if you don't have the space or money for a large rug, a small 2x3 by your bed will make all the difference. This bathmat is soft and luxurious enough to live in a bedroom as well.
3. Desk lamp: A lamp (or two) will keep you from straining your eyes during those all-night study sessions, and it will also make your place a lot cozier when you want to hang out at home without the harsh glare of fluorescent overhead lighting. This brushed steel droplet lamp is a great option, as is this cheery little yellow lamp.
4. Closet Storage Materials: You'll need some hangers, and I recommend a set of slim-line hangers since you probably won't have a ton of space in your college closet. Other things that can come in handy are jewelry organizers, modular storage cubes, or shoe pockets, but I really recommend waiting to purchase these items until you can see your closet space in person. It's much easier that way to figure out a system that will fit in the space that you have.
5. Kitchen Basics : My mini-fridge was a lifesaver when I was in college. I'd stock up on food during dining hall runs, and then I'd have plenty for times when the dining hall wasn't open or when I was too busy to go. A microwave was the other item that I used almost daily. Aside from that, have a plate or two, a couple of drinking glasses, a set of food storage containers (I love my IKEA Pruta set), and a small set of silverware. Some of my friends kept coffee machines or toasters in their rooms, but I never needed them, so go with what you think you'll be using most often.
6. Towels and Bath Caddy: Communal bathrooms can be a bit of a shock to the system, and while I can't prepare you for the types of weird behaviors you'll probably encounter, you can be prepared item-wise if you have a couple of towels (I like these soft, quick-drying towels), a bath caddy to tote your toiletries back and forth for showers, and depending on how (un)tidy your dorm-mates are, a pair of flip flops to wear while showering. The last item may sound alarmist, but trust me, there were some days that I was glad to not be stepping in the hair and grime of ten other girls.
7. Laundry Bag: I really like this canvas duffle bag's classic look and easy-to-carry design. Another good option is this pop-up hamper that has side pockets to carry your detergent and laundry sheets.
8. Cleaning Supplies: The bright side of a small space is that you won't have need for a lot of cleaning supplies. A Swiffer, broom, or mini stick vacuum should do the trick. And keep some disinfecting wipes, dust cloths, and magic erasers around for some quick tidying. As an added treat, I kept a bottle of this linen spray around, which doubled as an air freshener and kept things smelling clean.
9. Fan: This will depend on where you go to school obviously, but as a college student living in a dorm without air conditioning, you could barely pry me away from my clip-fan.
10. Artwork: This is where your space can really, truly become your own. Forgo the stereotypical college posters and start your own collection of art. There are some really affordable options on Etsy. Right now I'm loving the work of Vivienne Strauss, Clare Elsaesser, and, for a bit of fun, Angela Rossi. You may not be able to use nails or screws if you have cinder-block walls like I did, but Command strips or hooks are generally sufficient enough to support the weight of a basic frame. Frames will really elevate the look of your room, taking it from traditional college to homey chic.