By: Rabbi Ari Enkin
A number of halachot relating to conversion are learned from the book of Ruth which is one of the reasons it is read on Shavuot. The Talmud teaches that before Ruth’s conversion, Naomi told her: “We have rules as to where we can and cannot walk on Shabbat, rules regarding our dealings with the opposite sex, we have six hundred and thirteen challenging commandments to uphold, and we are strictly forbidden to worship idols.” After hearing these rules came forth Ruth’s famous response: “Where you walk, I shall walk; where you sleep, I shall sleep; your people are my people, and your God is my God.” It is from here that the Talmud rules: “We inform prospective converts of a few of the less serious commandments and a few of the more serious commandments. We do not overburden the convert with numerous commandments, nor with their fine details.”
Another halacha derived from the story of Ruth is the practice of greeting each other with the name of God, as Boaz himself did, as it is written: “Boaz came from Bethlehem and he greeted the reapers with ‘May God be with you,’ and they responded, ‘May God bless you.’” We fulfill this teaching today through the greeting “shalom aleichem,” and its response “aleichem shalom.” Shalom is one of God’s names.
Megillat Ruth is also the source  for the Talmud’s ruling that, “One may not leave the Land of Israel to go abroad unless the price of wheat has risen…but if one can still purchase wheat, although somewhat costly, one may not leave.” As Rabbi Shimon bar Yochai used to say: “Why were Elimelech, Machlon, and Chilyon, the greatest scholars and leaders of the day, punished? Because they left Eretz Yisrael even though wheat was available, albeit at a high price.” Nevertheless, the halacha is not in accordance with this view and leaving the Land of Israel is permitted in a number of situations – a discussion of which is beyond the scope of this post. 
Finally, and not widely known, is that the source for washing in preparation for Shabbat, along with the custom of wearing one’s finest clothing on Shabbat derives from the book of Ruth, as well. As Naomi tells Ruth: “Wash yourself, anoint yourself, and put on your fine clothes.” The Talmud comments on this verse, explaining: “These were her Shabbat clothes. Rabbi Chanina said: A person must have two sets of garments, one for weekdays and one for Shabbat.” In fact, the use of fine perfumes in honor of Shabbat was a custom of even the greatest sages, and is certainly a meritorious custom one should consider emulating.