If you are applying now, you must also sign up with LSDAS (Law School Data Assembly Service). The form and detailed instructions are in the LSAT/LSDAS Registration and Information Book. Note that as part of the process, you must arrange to have your college transcript (or transcripts) mailed to LSDAS.
IMPORTANT: If you just want to take the LSAT but you are not applying to law school this year, do sign up for the LSAT but do not register with LSDAS. You'll be wasting your money.
Although you must tell LSDAS how many law schools you are applying to (and pay accordingly), you do not need to tell LSDAS the names of those schools. Instead, when you apply to a particular law school, you will enclose a "Law School Application Matching Form," found in the back of the LSAT/LSDAS Registration and Information Book. Upon receipt of your application, the law school will provide your Matching Form to LSDAS, who will then supply the law school with a copy of your LSDAS report.