Lots of people do fine with one kidney, as long as it's a healthy kidney. The remaining kidney is usually able to get the job done.

The kidney filters our blood, taking out wastes, balancing the pH, and regulating water and salt. If it only goes through the kidney half as often (because there's only one kidney), it still gets filtered.

The body only needs 1 kidney, but it doesn't have a spare kidney if one is donated. Here are 2 links for people who are thinking of donating a kidney: donating a kidney.
Living kidney donor.

Be aware of the risk and weigh it against your decision to donate. Possible long-term risks to donating a kidney include hyper-tension (high blood pressure), hernia, organ impairment and the need for organ transplant, kidney failure, and death.

Although a single kidney can filter your blood well enough to keep you alive, you won't be healthy with only one kidney. Don't do it!