Go to your local high school. See if they offer an adult education program that includes a class to earn your diploma. Find out the details, as some states only offer this option up to age 21.
Take an online class. Earn your diploma in the comfort of your own home.You may do this either by correspondence courses or you may enter into a virtual classroom online.You are required to be at least 16 years old, but then the sky is the limit. Check the resources made available here for choices of schools to attend.
Earn your GED -- General Educational Development Test -- as another option. You must be 19 years or older and have not graduated from high school. Locate a testing center in your area by going to the following link -- http://www.acenet.edu/resources/GED/center_locator.cfm -- and entering your ZIP code. Call or go into the center to make an appointment to take the test. Have an issued picture identification to bring with you.