a single good deed

"One can't begin by teaching a beginner 613 laws or encouraging him to commit to several new practices all at once. Rather encourage him to take on a single good deed.  Ensure that he knows this is one of many good deeds about which he will learn at a later stage. Right now, however, focus on this one good deed. And then we have God's promise that when he will do that deed, it will have tremendous success. Rabbi Saadya Gaon taught, repeated by the Baal Shem Tov, quoted many times in Chasidic teachings, "When you grasp a part of the essence, or a small amount of it, you grasp the entire thing." By encouraging a Jew to observe one of the 613 commandments, it is in fact "a part of the essence." So ultimately it influences that Jew, such that he becomes connected with the essence. Or as the Mishnah puts it, one mitzvah leads to another and the Jew continues to increase in the light of Torah and mitzvos. Moreover, ultimately, this Jew himself teaches others, bringing other Jews to Judaism. He "expels nations" and implants the Jewish vine after banishing non-Jewishness from his own portion of the world. And when many Jews join together, each one transforms the materialism of his own portion of the world and "You implant it." This becomes God's plant. His handiwork in which He takes pride." We go to greet the true and complete redemption when all mankind "will serve God with one purpose." the entire world will be affected to serve God by performing their commandments. And the Jewish people radiate to the nations an eternal light through holding the light of Torah and the candle of mitzvos." 

7th Lubavitcher Rebbe, JEM, New World, Old Song, 840