I spend WAY too much time looking at design sites, so I have lots of ideas for places you could look for inspiration. A lot of what's coming down the design pike now is very young and fresh-looking -- translation: totally appropriate for your age!
With all due respect, I'm not sure good design has an age minimum or maximum. Your decor reflects your personality. Regardless of how old you are, I don't think I could recommend sites or sources without at least some idea of who you are and what you like. A 'young adult' can have modern or retro or elegant or beachy or traditional or transitional or whatever taste.
I can advise you to consider how you use the room (function) and how you can express those functions creatively (form). What do you need the room to do for you? How do colors, fabrics, furnishings, lighting, floor coverings, case goods, paint, accessories, etc. serve that or enhance that?
What are you allowed to do based on rental or parental rules? That will play a role. So will your budget.
At my advanced age, with a whole house to decorate myself (sorta - I share it with a husband and a cat), my ideals are something like this:
1. I love rugs, but they're possibly the biggest challenge. That's why I look for rugs first, rugs that express or embody what I want the room to be, then work with the rug for the other stuff. Ebay has been the best source of rugs for me.
2. Color is a big deal. Paint is easy, cheap and so effective. If that's all you can do, do it. It creates amazing changes. Don't bother with trim or ceilings; just paint the walls and watch the magic happen.
3. Fabrics are interchangeable, unless it's upholstery. That you have to live with. Pillows and bedding and whatnot you can change with your mood or the season.
4. Don't mix metals. Doorknobs and drawer pulls and lighting fixtures and hinges should be all the same family within the same room. These elements combine function and decoration, and you want them to harmonize and tell a story about the personality and character of the room and of you. (Usually you're a chrome/nickel/silver person or a brass/bronze/gold person.)
5. Mopping, dusting and vaccuuming are for losers. Choose whatever you can with maintenance in mind. I hate clutter but I'm OK with dust or dirt on surfaces that can handle it through camouflage. It's a given that my cat will run to the nearest rug when she feels the urge to vomit, so light-colored rugs are out, for example.