I need help to mark the meter.I grant I never saw a goddess go: My mistress, when she walks, treads on the ground. And yet, by heaven, I think my...

Shakespeare's primary meter through all of the sonnets is
iambic pentameter.  Each iambic "foot" is made up of one unstressed syllable followed by
one stressed syllable.  In a perfect line of iambic pentameter, there would be 5 iambic
feet.  In the section of the poem in your question I will use
bold lettering to note the stressed
syllable.

readability="15">

I grant I
never saw a
goddess
go:

my
mistress, when she
walks ,treads on the
ground .

and
yet, by heaven, I
think my love as
rare

as
any she
belied with false
compare.

When you look
at the words or syllables of words that are bolded you can notice that sometimes there
are sound connections within those words.  Shakespeare used alliteration, assonance and
consonance to further connect the words within a line.  For example, in the first line
above, 3 of the bolded syllables START with a "g" which is an example of alliteration. 
The vowel sound of "saw" and "god" both have the "a" sound which is an example of
assonance.  The last line above repeats the "l" sound in "belied" and "false" which is
an example of consonance.  It is clear that Shakespeare was intentionally creating some
masterful poetry with these lines, and in all of his writing.