The LSAT is a half-day exam consisting of 5 sections and a written essay.  The subject areas tested are reading comprehension, analytical reasoning, and logical reasoning.  The test is given every October, December, February, and June at locations throughout the country.  The test is designed to measure those factors thought to contribute to success in law school by testing an applicant's reading and verbal reasoning skills.  Although the usefulness of this standardized exam has been questioned, the LSAT is used heavily in deciding who becomes a law student and who doesn't.  There are various prep courses that are available and claim to help a candidate improve her LSAT score.  The jury is still out on whether or not these courses improve test scores.  There is also much debate about whether or not one can prepare at all for such an exam.
    The LSAC web site mentioned previously lists the specific LSAT test dates.  Generally however, your application must be in approximately 40 - 50 days prior to the test date you select.  Results are given out 2 1/2 to 5 weeks after the exam, depending on whether or not you pay an additional fee for early access to your score. 
    While some manner of preparation is a necessity, the degree to which you prepare depends largely on the time you have to commit, and the money you have to spend.  Preparation possibilities run the gamut from buying some prep books on your own and studying at home to attending live lecture courses such as Kaplan.  Be advised, live lecture courses can be very expensive.  You will find a shelf full of preparation materials by Barron's, Princeton Review and others at the larger booksellers such as Borders or Barnes & Noble. Your university's bookstore will also carry LSAT preparation books. 
    Also, consider the less mainstream LSAT prep options to see if they might serve you well.  For instance, noncredit community enrichment courses often include prep classes for the SAT, GRE, and LSAT.  These LSAT classes are usually reasonably priced, oftentimes include text and CD-ROM, and are frequently taught by a local lawyer or a 3L if you live in a town with a law school.