You can contact your closest community college or adult education center.  Most of them offer free classes to help prepare for the the 5 GED tests you take in reading, writing, math, science and social studies.
 
There is a cost to take the tests, which must be taken in person at one of the official testing facilities - usually at the same place you take the classes.  The cost varies from state to state but is general $100 or less.
 
Some states also offer online classes to help you prepare for the tests.  There are also a couple of free non-profit GED online classes.  You can also use free online practice tests to help you prepare.
 
For info and links to your state's GED info and the free classes and practice tests, look here:
 
http://www.squidoo.com/ged-websites