If you need a copy of your high school transcripts, contact someone in the administrative offices at the school to request a copy by mail. Sometimes school districts will have official request forms, which may be available online. If transcripts are being sent to a college for admissions, you should have them notarized or they will most likely not be valid or accepted.
You will have to provide proof of your identity. And there will probably be a cost associated with the request, which you may have to pay with a money order or cashiers check.