lightening very dark hair

If you only want to lighten two shades, and you don't have any dark dye in your hair, then you can use dye to lighten it. If you want to go blonde, you need to bleach the heck out of it, but to go from dark brown to medium brown is not too bad. If you do have dark permanent dye in there anywhere, use a remover such as ColorFix first, then you can dye.

Be aware that there is a good chance your hair will go reddish if lightened. If that's what you want, then great! If not, then use an ash dye (likely something with a green or blue base).

If you go to a beauty supply store to get your dye, you might consider using 30 vol. developer (box dyes usually use 20). The 30 vol. will give you a little extra lift. The general rule of thumb is 20 vol. will lift 1-2 levels, and 30 vol. will lift 3 levels. It's not an absolute, but it's a decent guide.

Also, dyes at the BSS usually tell you the base color, which is helpful. I like the Wella Colorcharm permanent line because they usually have the same level shade in a wide variety of base colors. To lighten to a neutral brown, use a blue-green base (or blue or green...depends what they have), for a golden brown, use a neutral base (the red/orange pigment in your hair will give it a warm cast), and for auburn, use a red base.

You could also take a look at the L'Oreal Hi-Color line, which is meant for dark hair (this works best on virgin hair).