first, you must make sure it is okay with your guardian or roommate, or whoever you live with that you are rearranging your room. you can ask for their opinion, but don't feel obligated to follow it, ultimately it's your room!
establish a budget and basic plan but again don't feel obligated to follow it because you may develop better ideas while you shop or rearrange.
Step 1: Shop around
go to different thrift stores and yard sales to see what they have. you'd be surprised to find some have some very cute stuff! I got my desk from a second hand store and I love it. (it's kind of messy right now, I need to clean it off!)
Step 2: Paint!
bored of the dull colours in your room? pick one wall you will paint bright, vibrant colours. try splattering paint on your wall by applying the colour you want and flicking the brush at the wall, or if you're very ambitious, cover all the furniture in your room and throw paint from the bucket!
Step 3: Be creative!
take furniture you already own and put it in a different place in your room. try putting your bed in a corner (make sure it's near an electrical outlet!) to open up the room and put your desk facing a window, so you have a nice view while you work! make sure the heights of the objects are either drastically different or the same, otherwise it looks strange. also, make it sensible: don't crowd or place objects illogically, make it functional but stylish at the same time. remember, your room is where you spend a lot of your time, so it's important that it is based on your interests and functions, not anyone else's!
Step 4: Test!
after you've rearranged your room, live in it for a few days experimentally: try to use everything and make sure it makes sense. if not, try again!