I've dyed my hair a dark brown and now I want to go to a lighter color what should I do

The first step you should take is go to a light brown. Something that you can tell is lighter the second your hair is done. Don't go too blonde, because then you're going to look like a goof. Just go with a light brown then take your color to something you think would fit you really good

You will need to use some kind of hair lightening product in order to go lighter. If you want to go a couple of shades lighter you can use a high-lift color or your selected hair with a higher volume peroxide developer to lift the color. However, since your hair is dark these techniques could result in a red/orange shade and you may need to use an ash base hair color to neutralize the unwanted tones. But if you want to go 3-4 shades lighter, it's best to bleach your hair to a level that's lighter than the color you want then apply the desired color. I recommend that you get it professionally done or ask someone that's experienced in hair coloring to help you color your hair.