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!)