click on the link for more details:

http://www.ctc.ca.gov/email/substitute/day-to-day.html

Requisites for subs vary depending on the state and school district. Some require a high school diploma, two years of post-secondary ed, a bachelor’s degree in any subject (this is the most common), and some even require a formal teaching license, though this is rare. If you’re interested in subbing in California, apparently you need a bachelor’s degree in any subject. In most states there’s also a thorough application process which may require basic skills exams, health screenings and immunizations, criminal background checks, and fingerprinting. But it’s all quite worth it–subbing can be a lot of fun, and you get to know all the different schools (and therefore all the children) in one schools district! And when you’ve had a great day, you can keep that class in mind. And when you’ve had a bad one, you can say “no, thanks,” next time they call you for that class. When you’re sick or just don’t feel like working, no problem (unless you do this a lot—teacher registries can also put you on the lazy list and stop calling you if you take off too many days). It’s really a fun, flexible job.