Your academic transcript is the most official documentation of your academic standing that exists, though really it's just a summary of your courses and grades printed on nice paper. When you apply to colleges and universities it will be required that you submit your transcript along with the rest of your application. Submitting a report card won't suffice.
Making a Transcript Request
Your high school will most likely be able to help you with the process of sending out your high school transcripts. Talk to your counseling office and let them know the schools you're planning on applying to. Give them the dates of each school's application deadline and let them know who they should send your high school transcripts to.
If your school does not have a system in place for sending out high school transcripts, you should be able to submit a transript request to get a copy - or copies - of your transcript from the school's registrar or Dean of Students so you can send them yourself.
Generally, you'll be sending two different transcripts to the school you're planning on attending. The first will be sent to them when you apply and will show your grade in each completed course since freshman year. The second transcript you send will again include each completed course grade since freshman year, but it will also be marked with a graduation date, indicating that you've earned your high school diploma.
Make sure when making your transcript request that both forms will be given to you and that you get enough copies to send out to all your prospective schools.
Transfering Your Transcript
Students hoping to transfer from one institution to another will almost certainly be asked to submit a transcript from their previous school. This should come as no surprise, since it's only fair that the school you're transfering to will want to see what kind of student you are on a more in depth level than just your cumulative GPA.
The process of getting your transcript sent to the new school is simple in essence, though each school will have their own tendencies and finite procedures.
Go to your school's registrar's office and fill out a transfer request form. Some schools give you the option to have your transcript sent directly to a mailing address - there will be an intended recipient box on the page - but others require that you wait for the transcript to be sent to you before you mail it yourself.