Newbie needing some bleaching advice!

Because you have been dyeing it dark for many years you have to accept that white may not be possible. Dark dyes can stain, especially over bleached hair, and the staining is permanent.
Colour removers are the place to start, you will need to use at least three. If you can space them out and use fading treatments in between that will get as much dye as possible out.
Then you can start the bleaching process, which will take several months. You need to wait at least a month between bleaches, ideally longer, 6-8 weeks.
You need to use a lower volume peroxide on hair that is already bleached, so if you use a 30 vol peroxide the first time, you need to drop to 20 for the second. You can use bleach baths to get the last traces of yellow out, if your hair will go that light.
It is impossible to predict how many times you will need to bleach, due to all the layers of dye on your hair you are likely to get a gradient from yellow at the roots through oranges and reds to black/brown at the ends, which will be difficult to get out.
In order to tone to white you need to bleach to a level 10, palest yellow, the colour of the inside of a banana.
Coconut oil really helps to minimise damage, but what you are proposing is the most difficult and damaging process possible so some damage will be inevitable. You will need to lose some length, especially if the ends are very stained.

In all honesty, I would say don't do it. I think if your hair is lower back length, then that hair has been dyed lots of times and probably in worse condition than it seems (unless you always used direct/veggie dyes and not permanent). And as wicked pixie says, it's also very likely stained.

I did it I went from black to white, using 2 colour b4's and I had used home dyes for many years, after the colour remover I waited then I had some help I paid for a full head of highlights first then bleached from there, I found the coconut oil helped so much I used loreal maximum blonde bleach I found this to be less damaging than other bleaches iv used, then I used wells colour fresh 8.81 toner to cancel out the yellow tones, iv recently started to use the LA riche directions silver which was good value but found the wella one lasted much longer, its hard to comment on the bleaching without seeing the hair, I'm lucky I have my mums hair very healthy and silky, fast repairs I don't know how must be all the coconut oil an fish oil iv been eating over the years, you know your own hair if your concerned stop and treat it before attempting again, I'm going real short by summer so I wasn't to fussed about the ends which do generally dry out with bleaching. I kept all my pictures in order the first one was after using colur b4 xtra strength on black hair, then the second was after using another colour b4 a few days later, i think it works best this way, then im sorry i dont have one of the highlights, but i basically just bleached over and toned, it does take time so patience is the key, use a cheap toner like the directions if you need to go out to work ect before bleaching again, thats what i did.