What happens if you only have one kidney?

Answer 1

One fully functioning kidney has the ability to clear all wastes from the body (that two kidneys would have), since healthy humans use only about 20-25% of their kidney capacity).
Nothing, you will still function 100% fine. However, im not sure if it is still the same now, but you cant join the defence force with only 1 kidney.

Answer 2

As long as the kidney you have is functioning properly, it is possible to not even know you only have one. As anyone that has donated a kidney should be able to testify, if it were critical to have two, you would not be able to donate one while you are alive.