מתני׳ מאימתי קורין את שמע בערבין. משעה שהכהנים נכנסים לאכול בתרומתן עד סוף האשמורה הראשונה דברי רבי אליעזר.
The beginning of tractate Berakhot, the first tractate in the first of the six orders of Mishna, opens with a discussion of the recitation of Shema, as the recitation of Shema encompasses an acceptance of the yoke of Heaven and of the mitzvot, and as such, forms the basis for all subsequent teachings. The Mishna opens with the laws regarding the appropriate time to recite Shema:

MISHNA: From when, that is, from what time, does one recite Shema in the evening? From the time when the priests enter to partake of their teruma. Until when does the time for the recitation of the evening Shema extend? Until the end of the first watch. The term used in the Torah (Deuteronomy 6:7) to indicate the time for the recitation of the evening Shema is beshokhbekha, when you lie down, which refers to the time in which individuals go to sleep. Therefore, the time for the recitation of Shema is the first portion of the night, when individuals typically prepare for sleep. That is the statement of Rabbi Eliezer.
וחכמים אומרים עד חצות.
The Rabbis say: The time for the recitation of the evening Shema is until midnight.
רבן גמליאל אומר עד שיעלה עמוד השחר.
Rabban Gamliel says: One may recite Shema until dawn, indicating that beshokhbekha is to be understood as a reference to the entire time people sleep in their beds, the whole night.
מעשה ובאו בניו מבית המשתה אמרו לו לא קרינו את שמע אמר להם אם לא עלה עמוד השחר חייבין אתם לקרות ולא זו בלבד אמרו אלא כל מה שאמרו חכמים עד חצות מצותן עד שיעלה עמוד השחר
The mishna relates that Rabban Gamliel practiced in accordance with his ruling. There was an incident where Rabban Gamliel’s sons returned very late from a wedding hall. They said to him, as they had been preoccupied with celebrating with the groom and bride: We did not recite Shema. He said to them: If the dawn has not yet arrived, you are obligated to recite Shema. Since Rabban Gamliel’s opinion disagreed with that of the Rabbis, he explained to his sons that the Rabbis actually agree with him, and that it is not only with regard to the halakha of the recitation of Shema, but rather, wherever the Sages say until midnight, the mitzva may be performed until dawn.
גמ׳ תנא היכא קאי דקתני מאימתי
GEMARA: The Mishna opens with the laws concerning the appropriate time to recite Shema with the question: From when does one recite Shema in the evening? With regard to this question, the Gemara asks: On the basis of what prior knowledge does the tanna of our mishna ask: From when? It would seem from his question that the obligation to recite Shema in the evening was already established, and that the tanna seeks only to clarify details that relate to it. But our mishna is the very first mishna in the Talmud.
תנא אקרא קאי דכתיב בשכבך ובקומך והכי קתני זמן קריאת שמע דשכיבה אימת משעה שהכהנים נכנסין לאכול בתרומתן
The Gemara offers a single response to both questions: The tanna bases himself on the verse as it is written: “You will talk of them when you sit in your home, and when you walk along the way, when you lie down, and when you arise” (Deuteronomy 6:7). By teaching the laws of the evening Shema first, the tanna has established that the teachings of the Oral Torah correspond to that which is taught in the Written Torah. And based on the Written Torah, the tanna teaches the oral law: When is the time for the recitation of Shema of lying down as commanded in the Torah? From when the priests enter to partake of their teruma. Just as the Written Torah begins with the evening Shema, so too must the Oral Torah.
ואי בעית אימא יליף מברייתו של עולם דכתיב ויהי ערב ויהי בקר יום אחד
However, there is another possible explanation for why the mishna opens with the evening Shema rather than with the morning Shema. If you wish, you could say instead that the tanna derives the precedence of the evening Shema from the order of the creation of the world. As it is written in the story of creation: “And there was evening, and there was morning, one day” (Genesis 1:5). According to this verse, day begins with the evening and not the morning. For both of these reasons it was appropriate to open the discussion of the laws of the recitation of Shema with the evening Shema.

(א) תְּפִלַּת הַשַּׁחַר, עַד חֲצוֹת. רַבִּי יְהוּדָה אוֹמֵר, עַד אַרְבַּע שָׁעוֹת. תְּפִלַּת הַמִּנְחָה עַד הָעֶרֶב. רַבִּי יְהוּדָה אוֹמֵר, עַד פְּלַג הַמִּנְחָה. תְּפִלַּת הָעֶרֶב אֵין לָהּ קֶבַע. וְשֶׁל מוּסָפִין כָּל הַיּוֹם. רַבִּי יְהוּדָה אוֹמֵר, עַד שֶׁבַע שָׁעוֹת:

(1) Shacharit [can be said] until midday. Rabbi Yehudah says until four hours into the day. Minchah [can be said] until the evening. Rabbi Yehudah says until the middle of the afternoon. Ma'ariv has no set time and Musaf can be said all day. Rabbi Yehudah says until seven hours into the day.

(ג) רַבָּן גַּמְלִיאֵל אוֹמֵר, בְּכָל יוֹם מִתְפַּלֵּל אָדָם שְׁמֹנֶה עֶשְׂרֵה. רַבִּי יְהוֹשֻׁעַ אוֹמֵר, מֵעֵין שְׁמֹנֶה עֶשְׂרֵה. רַבִּי עֲקִיבָא אוֹמֵר, אִם שְׁגוּרָה תְפִלָּתוֹ בְּפִיו, יִתְפַּלֵּל שְׁמֹנֶה עֶשְׂרֵה. וְאִם לָאו, מֵעֵין שְׁמֹנֶה עֶשְׂרֵה:

(3) Rabban Gamliel says: Every day a person must pray eighteen [blessings of Shemoneh Esrei]. Rabbi Yehoshua says: [One may say] an abbreviated [form of the] eighteen [blessings]. Rabbi Akiva says: If his prayer is fluent in his mouth, he must say eighteen; and if it is not -- an abbreviated eighteen.

(ד) רַבִּי אֱלִיעֶזֶר אוֹמֵר, הָעוֹשֶׂה תְפִלָּתוֹ קֶבַע, אֵין תְּפִלָּתוֹ תַּחֲנוּנִים. רַבִּי יְהוֹשֻׁעַ אוֹמֵר, הַמְהַלֵּךְ בִּמְקוֹם סַכָּנָה, מִתְפַּלֵּל תְּפִלָּה קְצָרָה. אוֹמֵר, הוֹשַׁע הַשֵּׁם אֶת עַמְּךָ אֶת שְׁאֵרִית יִשְׂרָאֵל, בְּכָל פָּרָשַׁת הָעִבּוּר יִהְיוּ צָרְכֵיהֶם לְפָנֶיךָ. בָּרוּךְ אַתָּה ה' שׁוֹמֵעַ תְּפִלָּה:

(4) Rabbi Eliezer says: One who makes his prayer "set" [as though it is burdensome to him], his prayer does not constitute "pleading" [for Divine mercy]. Rabbi Yehoshua says: One who is traveling in a dangerous place should offer a brief prayer [and] say: Save, G-d, Your people, the remnant of Israel; at every period of transition let their needs be before You. You are the Source of all blessing, G-d, Who heeds prayer.

(ה) הָיָה רוֹכֵב עַל הַחֲמוֹר, יֵרֵד. וְאִם אֵינוֹ יָכוֹל לֵירֵד, יַחֲזִיר אֶת פָּנָיו, וְאִם אֵינוֹ יָכוֹל לְהַחֲזִיר אֶת פָּנָיו, יְכַוֵּן אֶת לִבּוֹ כְּנֶגֶד בֵּית קֹדֶשׁ הַקָּדָשִׁים:

(5) If one was riding a donkey, he should dismount from it [while he prays]. And if he is unable to dismount, he should turn his face [towards Jerusalem]. And if he is unable to turn his face, he should focus his heart toward the Holy of Holies [in the Temple in Jerusalem].

(ו) הָיָה יוֹשֵׁב בִּסְפִינָה אוֹ בְקָרוֹן אוֹ בְאַסְדָּה, יְכַוֵּן אֶת לִבּוֹ כְּנֶגֶד בֵּית קֹדֶש הַקָּדָשִׁים:

(6) If one was sitting in a boat, or in a wagon, or on a raft, [when he prays] he should focus his heart toward the Holy of Holies.

(א) אֵין עוֹמְדִין לְהִתְפַּלֵּל אֶלָּא מִתּוֹךְ כֹּבֶד רֹאשׁ. חֲסִידִים הָרִאשׁוֹנִים הָיוּ שׁוֹהִים שָׁעָה אַחַת וּמִתְפַּלְּלִים, כְּדֵי שֶׁיְּכַוְּנוּ אֶת לִבָּם לַמָּקוֹם. אֲפִלּוּ הַמֶּלֶךְ שׁוֹאֵל בִּשְׁלוֹמוֹ, לֹא יְשִׁיבֶנּוּ. וַאֲפִלּוּ נָחָשׁ כָּרוּךְ עַל עֲקֵבוֹ, לֹא יַפְסִיק:

(1) [One] should not stand up to pray unless he is in a serious frame of mind. The original pious ones used to wait one hour and then pray, in order to direct their hearts towards the Omnipresent. [While one is reciting Shemoneh Esrei,] even if the king greets him, he should not respond to him, and even if a snake wraps around his heel, he should not interrupt.