Because the Law School Admissions Test is integral to the law school application process, students looking to attend law school the fall after graduation should start LSAT preparation no later than spring of junior year.
To assess your need for practice for the test, it would be wise to take a scrupulously timed full practice test. The method of preparation you select will depend upon your unique personal circumstances, but you should not take the test without adequate preparation using either LSAC materials or reputable test preparation companies. Expect to devote adequate (ideally no less than eight weeks) to prepare.
To help you discern how much preparation you need, you would be well served to check out the low cost materials the LSAC offers such as Sample Questions with Explanations and free test prep videos as well as official prep materials such as disclosed LSAT tests and Preptest Ebooks.