BRACHOS 2a

'Why does the Mishnah choose the reading of Sh’ma at night to be the very first halacha to be discussed in Shas?'
Tosafos (Shabbos 2a) notes that many masechtos (tractates) are arranged according to the order in which the events of a particular day occur. For example, Massechta Pesachim begins with the laws of bedikas chametz, which is a full day before the festival of Pesach actually begins.

The massechta then deals with the slaughtering of the korban Pesach, which is the afternoon before the chag, and it finally progresses to the holiday and its laws. Yoma also begins with the preparations of the Kohen Gadol a week before Yom Kippur, and it progresses sequentially through the actions of the Kohen the day before Yom Kippur, and finally we are introduced to the service of the day itself.

Similarly, here, when a person becomes barmitzvah, his obligation to fulfill mitzvos begins as night falls, and the first mitzvah anyone ever has the obligation to fulfill is the reading of Shema that night, as he concludes his thirteenth year and enters into his fourteenth year.
In fact, the verse itself is structured to reflect this, as the Torah instructs us to read Shema “as you lay down and as you awaken.” (Devarim 6:7)