This is a large topic, but for your homework you will want to talk about aldosterone, produced by the adrenal glands, and ADH or vasopressin, from the pituitary...here is a reasonable summary I found, plus a couple of links.
"The brain and kidneys regulate the amount of water excreted by the body. When the blood volume is low, the concentration of solutes in the blood is high. When the hypothalamus senses this low blood volume and increased serum osmolality it synthesizes ADH, a small peptide molecule. The pituitary gland then releases ADH into the bloodstream and causes the kidneys to retain water by concentrating the urine and reducing urine volume."