Why do my knees crack wen I get up?

Fluid buildup. Most joint noises are attributed to a hydraulic effect of fluid in the joint (normal or increased quantity) being forced around based on movement. This is akin to the bang or rattle heard in pipes in older houses with abrupt water turn-off. It can indicate excess fluid--like from injury, or be normal. It is usually harmless, but should be evaluated if associated with pain or decreased motion.
Many possibilities. Sound coming from the knee could be due to tendon, cartilage, meniscus, or bone abnormalities. Have it evaluated by your orthopedist and find out why it is making that sound.