Short answer: Ask your school counselor for a fee waiver. Then, when you’re registering for the SAT, enter some numbers in the online registration form.
More information: First, to get a fee waiver, visit your high school guidance counselor. She or he should be familiar with the process and will give you a card with some numbers on it.
Second, when you go through the long process of registering for the SAT, and you finally arrive at the payment screen, you’ll see an option similar to this (I’m giving a summary here; I don’t want to get on the wrong side of College Board!):
Do you have a [sic] SAT Fee Waiver: Yes No
And if you click the tooltip (the little i (for “information”) image), you’ll see information telling you the following:
Visit your high school guidance counselor to request a fee waiver if you or your family cannot afford to pay the SAT registration fee or fees. Fee waivers entitle you to several College Board services for free and cover the basic registration fee for the SAT (the Reasoning Test) or for the SAT Subject Tests. Fee waivers also cover the cost of the SAT Question-and-Answer Service (QAS) or the SAT Student Answer Service (SAS).
You will then be asked to enter your SAT Fee Waiver Identification Number (ID). The fee-waiver ID is basically the number from your fee-waiver card, plus your school’s six-digit code. So, look for a number on your fee waiver card. To that number, add your school’s six-digit code.
Finally, the fee waiver ID number is unique and can only be used once, so no, you won’t be able to use this particular fee-waiver as many times as you want. To get another fee waiver, you’ll need to see your counselor again.
Important! You’ll notice that the fee waiver entitles you to the QAS or the SAS, for which there is normally a charge. Since it’s free, you should definitely request these!