I have been dying my hair blonde for years and want to go a nice chocolate brown, I've heard I need to pre pigment first but what do I use?

Prepigmenting isn't the problem here, what is the problem, you will not get a nice chocolate brown, but a deep black because your over processed hair is very porous.

I would suggest a colour two or three shades lighter then what you want. I don't know how long your hair is, but would suggest as well, after colouring, having some cut off. And start conditioning.