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Decide what color(s) you want to dye your hair. Figure out which product you want to use. Look for reviews to make sure you get a quality brand.  Some shampoo/conditioner brands also sell hair dye. It may help your case to pick a brand your parents use to wash their own hair. If your parents are vegan or against animal testing, consider picking a dye that's vegan or doesn't test on animals. Hair dye contains hydrogen peroxide, which can cause your hair to turn dry and brittle. It's unlikely that dyeing your hair once will cause any sort of serious damage, but be aware that it may still cause some damage, especially if you are going from dark to light.  The chance of you being allergic to hair dye is low, but it's still worth doing a patch test before you apply the dye to your head. Just put a small amount of dye on your wrist or ankle, and wait 24 hours to make sure there's no allergic reaction. There are many peroxide free hair dyes. They may not be as cheap as drugstore box dyes; however, they are much better for your hair. You don't want to get in trouble. If your school doesn't allow unusual colors, it's also very unlikely your parents will give you the okay. Be at the right age. If the package says, "Not recommended for use if under 16 years of age," do not use it as a 13-year-old. You could get serious hair follicle damage.
Do your homework on the hair dye. Learn about the risks involved in dying your hair. Make sure whatever you're doing is within the dress code for school/work.