My aunt gave me a bottle of “Diva Wash,” which is the soap made for it by the company that makes Diva Cups. As long as you find something mostly natural and fragrance-free, that’s pretty much the idea, I think. Diva Wash is plant-based, hypo-allergenic, and dye/fragrance-free.
I think it’s mentioned in the instruction booklet with the cup, but if not, note that you may have to trim down the little stem of the cup to be comfortable. I cut off a little bit of it, though I guess not everyone has to. Be patient with getting used to it, it’s so worth it.