The poem "The World Is Too Much with Us" is a sonnet divided into an octave and a sestet. Where does the sestet begin?

Concerning your question about Wordsworth's "The World is Too Much With Us," there's more to determining an octave and sestet and studying how the poem works than just counting lines.  In fact, in this Wordsworth poem the octave and sestet do not consist of eight and six lines. 

Poems contain lines of thought.  In this poem the line of thought turns in the middle of ninth line, rather than after the eighth.  Notice the semi-colon at the end of the eighth:  the sentence continues into the next line.  This is not conclusive, by any means, but when paired with the context it certainly provides evidence that the turn occurs with the exclamatory

...Great God!  I'd rather be

A Pagan suckled in a creed outworn;

So might I standing on this pleasant lea,

Have glimpses that would make me less forlorn;

Have sight of Proteus rising from the sea;

Or hear old Triton blow his wreathed horn.

In the first 8.5 lines, the speaker establishes that we ("us") are out of tune with nature and it moves us not, because other things get in the way.  That is the thought of the first 8.5 lines. 

The turn occurs with the "--Great God!"  The thought turns to the speaker insisting that he would, then, rather be an out of date pagan than be like people are today (in the poem's present), because at least pagans have a connection to nature. 

Those are the ideas presented in the poem, with a clear turn in the line of thought after the first 8.5 lines.