The reason the first Mitzvah begins with the letter Alef:^[1]

The letter Alef is the first letter of the Hebrew alphabet and represents the divine attribute of Keser. It is for this reason that the first commandment, which is the commandment of Anochi Hashem Elokecha, begins with the letter Alef. This comes to hint to the fact that through fulfilling the Mitzvah’s one draws down from the level of Keser.

[1] Torah Or 11b