Pirke Avot (the Ethics of the Fathers) says, find yourself a teacher, and that advice remains good. The best way is to study with a rabbi. Many rabbis offer introduction to Judaism courses. These are frequently seen as courses for potential converts, but taking such a course does not commit a person to conversion.Short of that, there are lots of good introductory textbooks. I personally enjoy Telushkin's encyclopedic works, Jewish Literacy and Jewish Values. Kertzer's What is a Jew has remained in print for well over 50 years (it's much smaller than Telushkin's works). Steinberg's Basic Judaism is a tiny little book, but for many people, a small bite is a good way to begin.