Appreciate your resolve to donate kidney
to your friend. Normally body will take care of itself. There is nothing you can do to cleanse your kidneys. Maintain adequate hydration. Drink at least 3 ltr of water daily. Avoid smoking
and alcohol before surgery. Avoid drugs which are toxic to kidneys most commonly pain killers. Don't take any medication without prescription.
Spots in the kidneys are most Likely calcium Plaques which may a precussor of stone formation. Check if you have a family history of stone formation.
Wishing you well