The difference between Tohu and Tikkun:

The main difference between Tohu and Tikkun is that the world of Tohu contained very high, sublime Divine lights with only small vessels to retain them. Tikkun, however, contains a balanced quantity of Divine lights with substantially large vessels to withhold the light. Thus, while the world of Tohu contained superior Divine lights, the world of Tikkun contains superior vessels. These vessels are rooted in a very high level of G-dliness and surpass the advantage of even the sublime lights of Tohu. This is similar to the power of intellect found in the brain in relation to the ability of movement found in the feet. When weighing the two soul powers against each other, the power of the mind is certainly of greater importance and prestige than the power of movement in the legs. Nevertheless, it is specifically the legs that can carry the mind to wherever it chooses. Therefore, the legs contain certain advantages even over the mind.

This difference is similar to the difference between the relationship between Torah and Mitzvos. While Torah is certainly higher than Mitzvos, being that it involves the mind and intellect as opposed to mere action, nevertheless the effect of Mitzvos reaches higher than Torah. This is vividly seen from the law that states that one must nullify his Torah learning to perform a Mitzvah that cannot be performed by others. This is because Mitzvos contain an advantage even over Torah. It is for this reason that even a simpleton who performs a Mitzvah without any Divine intent, but simply due to this being G-d’s will, can find more favor in Hashem’s eyes than the great Kabbalistic intentions and unification performed by a Tzaddik. All of this is due to the advantage contained within the vessels of Tikkun over the lights of Tohu.

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