Black to white

Very difficult and may even be impossible. If you are determined to be white and don't mind going through some less pretty colours along the way, you will need to do it gradually over the course of a few months.
The way to approach it is first get all the old black dye out with colour removers. You will need at least 3. What colour was your hair underneath the black dye? If it was bleached it is quite likely to have stained, which is usually permanent.
When all the old dye is out you can start lightening. It will take several goes to get to the palest yellow, it is best to wait at least a month and reconstruct in between bleaching. The strength of peroxide will depend on your starting colour, but as your hair gets lighter you reduce the level of peroxide (as bleached hair is more fragile) You need an even pale yellow/cream colour all over with no brassy orange traces to tone to silver/white.

If your hair isn't in good condition it is unlikely to survive this process. The best thing to do will be to cut off any damaged areas and start with less processed hair.

I know it will be a long process but it will be worth it and i probably wont start dyeing it untill my hair is healthier. My hair was blue before i dyed it black and it did stain cause you can still see the green trough.