Not to be picky, but you are actually looking to become LEED accredited, not certified. LEED certifies buildings and accredits people.
Having said that, I am a LEED AP and the process is fairly simple. You can do it in one step, taking two tests at once to be an AP, or in two steps, becoming a GA first and then an AP later.
Check out this page for more info: http://www.usgbc.org/leed/credentials
You an prepare by purchasing books through USGBC, using free material available online, and also by attending online classes available from a variety of sources.
It’s a great accredidation to have. Good luck!