You will need to contact your high school to see if they (and/or the school district) have a website that offers transcript ordering online. If the school is located in the United States and you don't have its contact info already handy, you can look it up in SchoolFinder: http://www.education.com/schoolfinder/
I'm also including links below to some additional college-related resources you may find helpful.
The good news is that it's actually a pretty simple process - I recently had to do this myself. Many high schools now let you order and pay (if necessary - sometimes you're allowed to order it a couple times for free) for transcripts online. If the high school you attended allows individuals to do this, visit the school website's section for Alumni, and you should find a link. Or you can try contacting a school administrator via email. If the school has a website, you can find it using Education.com's SchoolFinder feature - http://www.education.com/schoolfinder
If the high school you attended doesn't provide this option, you'll need to get the phone number for the school, or school district. Ask to speak to someone who keeps track of school records, and let them know that you're trying to get a copy of your high school transcript, as well as what year you attended school.
They should be able to mail you your transcript, and you should receive them within a week or two.
Have other questions about Getting Ready for College, or other topics in parenting and education? Feel free to ask them here: http://www.education.com/question/ask/ The forum is frequented by education & parenting experts, as well as dedicated members who provide great input!
Below, I've included a link to other questions in the Getting Ready for College topic, as well as several other related questions. You may find that other members received answers that can help you prepare for college, too.
I am about to start school this at Ivy Tech here in Indianapolis, Indiana and I really need my transcript from West Side in Gary, but my problem I do not have transportation to get up there and get it. How do I go by getting it?