What are the requirements to be a sperm donor?

It's quite strict. There are national guidelines for screening sperm donors in the US. Most reputable banks adhere to those guidelines. These include, and aren't limited to, being tested via sperm analysis as well as blood tests to assure good health and absence of infectious diseases and genetic disorders. Contact a reputable sperm bank (like California Cryo or Fairfax Cryobank) for further information.
Healthy college grad. Most sperm banks require good health, no significant family medical issues or psychiatric history, graduation from college and no genetic anomolies or sexually transmitted diseases.