Dying my hair lighter?

Your hair is naturally medium brown? Well that could mean a lot of things... I would suggest buying as light of a blonde dye as you can find, because it won't really do much if your hair is medium brown. Then once you do that, wait a week and a half or so, and dye your hair with a blonde dye again. Keep doing that until you get the shade that you want. If I were you, I would just not use bleach yet and see how light you can get it with just dye. If your hair is really resistant, THEN you can use bleach.