Shakespeare's Sonnet #12 (like many of his sonnets) explores the troubling fact that beauty does not last forever.
Shakespeare gives several examples of how time and age spoil beauty, including:
a) "the brave day" sinks into "hideous night";
b) "lofty trees" become "barren of leaves";
c) "summer's green [grass and grain] all girded up in sheaves [bundles of dull-colored hay]."
All of this leads the poet to question the beauty of his beloved, because even she "among the wastes of time" must eventually go.
I wouldn't say that you should recite the poem in a mournful tone. Rather, I would suggest a tone of philosophical resignation, a tone that suggests I know this is tough but this is how it is.
The end of the poem could be expressed with a touch of hopefulness, because there the poet finds one way out of the grip of time:
And nothing 'gainst Time's scythe can make defence
The word "breed" here refers to children; by creating children, a person can obtain a sort of eternity.
An important general note about reciting poetry: Do not pause at the end of a line, unless it ends with a comma or period. In this poem, that means that you should not pause at the end of line 13, because there is no comma or period or comma there.