Mostly it depends on how/why you lost the kidney. Never had one? Not much impact at all; the body is very good at adapting. Lost one to a trauma/injury/donation? Not much impact at all, again, the body adapts. You want to watch your fluid intake, and neither overload the single kidney with water nor allow it to become dehydrated, but that’s really about it.
Lost to kidney disease? Well, you may still need dialysis and medications, because the remaining kidney either has or is at much higher risk of also becoming diseased. Stones? Cancer? Same deal - the underlying condition doesn’t change and should still be treated.
The biggest risk is, of course, losing your kidney. You don’t have the natural back-up that the rest of us have, and while the body can adapt quite well to one kidney, it doesn’t do well at all with zero kidneys.
A dear friend lost one of her kidneys to a car accident 12 years ago. Once they found the sponge the surgeon left inside and denied for 3 months while it abscessed, she’s been fine. Even carried a full term pregnancy and delivered a perfectly healthy baby boy.