Determine whether you need to send transcripts yourself or if they need to be forwarded by your high school. If the institution provided you with a transcript request form, all you need to do is fill it out and send it to your high school, which will then take care of the rest.
Visit your school district's website. Some schools now have online transcript request forms. This could be a web based interface, or a just a PDF that needs to be downloaded, filled out, and sent to the school or district office for processing.
Locate the name and address of your high school's current guidance counselor.
Write a polite letter explaining who you are, and where you need the transcripts sent. Provide a phone number, should they need to contact you. The high school counselor will mail the transcripts for you.