Contact the school district's central records and make a request. There may be a fee for the copy. This is what is needed for job applications, etc. The diploma is usually handled through the school. If it has been more than five years since your graduation that may be a problem. Call the school and ask. Many keep unclaimed diplomas for a few years.
I suggest that you contact the school (or school district) directly to obtain your records. You may use SchoolFinder to research the school or district's telephone number: http://www.education.com/answers