Shemmitah Order

 The parsha opens דַּבֵּ֞ר אֶל־בְּנֵ֤י יִשְׂרָאֵל֙ וְאָמַרְתָּ֣ אֲלֵהֶ֔ם כִּ֤י תָבֹ֙אוּ֙ אֶל־הָאָ֔רֶץ אֲשֶׁ֥ר אֲנִ֖י נֹתֵ֣ן לָכֶ֑ם וְשָׁבְתָ֣ה הָאָ֔רֶץ שַׁבָּ֖ת לַי״יֽ.  Then it continues שֵׁ֤שׁ שָׁנִים֙ תִּזְרַ֣ע שָׂדֶ֔ךָ וְשֵׁ֥שׁ שָׁנִ֖ים תִּזְמֹ֣ר כַּרְמֶ֑ךָ וּבַשָּׁנָ֣ה הַשְּׁבִיעִ֗ת שַׁבַּ֤ת שַׁבָּתוֹן֙ יִהְיֶ֣ה לָאָ֔רֶץ.  Why would the Torah start by telling us the obligation of shmittah if it only follows after the six years?  And why does it tell us וְשָׁבְתָ֣ה הָאָ֔רֶץ, and only afterward explains when?  

It says in Berashis (2:2) ויכל אלקים בַּיּ֣וֹם הַשְּׁבִיעִ֔י מְלַאכְתּ֖וֹ אֲשֶׁ֣ר עָשָׂ֑ה וַיִּשְׁבֹּת֙ בַּיּ֣וֹם הַשְּׁבִיעִ֔י מִכׇּל־מְלַאכְתּ֖וֹ אֲשֶׁ֥ר עָשָֽׂה.  The Or Hachaim asks why does it say ויכל אלקים בַּיּ֣וֹם הַשְּׁבִיעִ֔י מְלַאכְתּ֖וֹ if it was completed on the sixth day abnd why does it need to at the end מכל מלאכתו, it is obvious it from the work?  The Or Hachaim explains that until Shabbat the world was incomplete.  The world became "settled" only when through Shabbat.  The entire creation was in essence completed in Shabbat.  That is what it means ויכל אלקים בַּיּ֣וֹם הַשְּׁבִיעִ֔י and the possuk ends מכל מלאכתו because all of the creation was completed through Shabbat.  According to the Or Hachaim it is Shabbat that gives the life force to the world.  

The possuk calls shemmitah שבת לה.  Shemmitah has an aspect of Shabbat to it.  Just as Shabbat gives life to the week, shemmitah gives the life to the shemmitah cycle.  That's why the Torah opens with the mitzvah of shemmtiah before the six years, because it is the mitzvah of shemmitah that allows for the six years of work.  Then it continues according to the order of the timeline, six years of work followed by the year of shemmitah.