First make sure your clothes fit you properly, that will instantly make them look better. For jeans, know that lighter washes are more casual and darker is dressier, so dont wear a dressy silk top with powder blue jeans. If they are trousers or shorts or something other than jeans, go for solid colors or muted prints. Sometimes a trouser can have a small pinstripe, or plaid in a shade lighter than the pant, and if thats the case, you CAN wear it with a printed top. Just make sure the tops print is larger than the pant. As for colors, I dont mix black, brown, navy blue together, I choose one, like black, and add color. Gray is flattering to a lot of people too. And you can always dress monochrome like black or gray from head to toe and just add a bright shoe and some brightly colored chunky accessories.
the following stores ROCK for this exact problem
Target (especially the GO collections)
White House/Black Market
all those stores have stuff that is perfect for that transition age. im like you, im 22, and i find myself looking at the juniors section and thinking that most of it wouldnt even look good on a transvestite circus clown. but the stores i have listed have enough stuff from both the juniors department and womens department that fits the transition age quite well. boston store has amazing tops and there is almost always a sale. H&M just freaking rocks. kohls is good for pants. Macys is always worth a look. Forever 21 often looks really young but if you look theres always a few peices that are really classy and can work for whatever occasion you can think of. Express is great and always trendy. Targets GO collections are often made by european designers so they are still young looking but much classier than the standard jeans and Tshirt look. and well, white house black market has really awsome dresses and skirts.
i cant link pictures on the computer im on but ive been at all the stores i just listed in the past month and i assure you they all have cool stuff. the best way to find clothes that work is to mix and match from juniors and womens. like, theres loads of tops in the juniors section right now that are to die for, but most of the skirts and things are just wicked short or trashy looking, so thats where you take a step into the womens section. or if you find a top in the womens section, find a pair of pants from the juniors section.
oh, and the only other thing i can suggest is try to pick stuff that can transition from normal day wear to a more formal look if you need it too. like, if your gonna buy a top, consider all the places you could wear it. the more places you could wear it the better.