Half of my arms are darker. I can't wear sleeveless dresses. What should I do?

There are many reasons why the skin darkens which may include sun exposure, inflammation, existing skin conditions, medication, genetics, friction, etc. There are many ways to lighten the skin including lasers and peels. IV therapy may also be helpful for you. It involves directly injecting a concoction of vitamins and minerals into your bloodstream. The concoction will be tailored by the doctor depending on your needs which for this case is skin lightening.

Without a photo it is difficult to determine why ; I am assuming sun damage.  If this is the case wear SPF 40 or higher and protective sun wear.  See a dermatologist to discuss how to even skin tone.Best of luck..