The Sefer Chagvei Haselah explains that the Torah starts with a beis indicating people created with different characteristic and the Aseres Hadibros which includes all the mitzvos of the Torah (see Kar Hakemach and Baal Haturm who says that is has 620 letters for 613 mitzvos and 7 bnei Noach) starts with an aleph to show that the Torah unites.
