While Sefer BeReishit does not contain any Mitzvot for Bnei Yisrael, we do use it as a source for a number of traditions. One such source is when Avram says that he is too old to have any children anymore, and Hashem tells him to go outside and look up at the star-filled sky. Hashem challenges him to count the stars in the sky and tells Avram that he will have as many descendants as there are stars. From this a wedding practice emerged: Young brides would often get married outdoors in hope of having many children while older couples, who do not plan on having children, would remain inside.
After Avram fulfills the Mitzvah of Brit Milah, Hashem promises to him all of the Land of Israel. This seems excessive, as no one, not even a large family, needs an entire country to himself. One can say that it's simply because Hashem knew that this land would later belong to an entire nation. Rav Yaakov Emden says we learn another custom from the timing of this gift. The act of giving gifts to the parents of the baby at a Brit Milah is derived from this Pasuk. God gave Avraham this gift shortly after his Brit, making the Land of Israel Hashem’s gift to Avraham upon his Brit Milah.
Rav Moshe Matt states that the Minhag of waiting until after the Brit to give the baby the name is also derived from Avraham's Brit Milah. Soon after Avram experiences his Brit, he receives the new name Avraham from Hashem. In Avraham's case, and in all subsequent Britot Milah, he was undergoing the process of becoming a Jew. Here it was fitting to give him the name "Avraham," which means patriarch of many nations. We too give new names at a Brit.
Avraham lived a life that truly reflected his being a Tzelem Elokim, the image of God; this was clearly shown when he fought a war recorded in BeReishit Perek 14. A massive conflict broke out and Lot was taken captive. Avram sprang into action, and, with only 318 men, he saved Lot and changed the course of the war. Afterward, he returned and did not accept any of the spoils of war. The Gemara says that in response to this, all of his descendants are privileged to have the Mitzvot of Tzizit and Tefilin. Every time we put on our Talit and Tefilin in the morning, we are reminded of the heroism of Avraham. Just like Avraham Avinu’s actions turned a secular action into a Mitzvah, so too should we turn the secular aspects of our lives into Mitzvot.
In each of these examples we should think about what lesson we can learn from the source of our practices. Whether we base something completely from the text, as in the practice to have outdoor weddings, or find a Remez (allusion) in a Pasuk, like with Talit and Tefillin, we should think about the connection being drawn. By finding deeper meaning in these traditions we can have a greater appreciation for many parts of our Judaism.
Adapted from a Devar Torah by Rav Hershel Schachter