Considering the title, the reader suddenly understands this is a sonnet contained in a collection of poems, surely the one addressed to the "Fair Youth".
Analyzing the layout, you can understand this sonnet follows the Elizabethan model. Indeed, it is organized in three quatrains and a rhyming couplet. So, the reader may be interested in finding out what the quatrains' problem is.
Right from the start, the speaking voice sends a message for thou (second person personal pronoun) to tell him that looking at himself, everyone can recognize that he is getting old. In order to do that, the poet makes a metaphor, comparing himself to the autumn. The comparison is suitable because old age will progress to death as the autumn will progress in winter.
The rhyme scheme is ABAB in the quatrains, AA in the rhyming couplet. Rhyme ties words together, creates a musical effect, adds unity and makes the quatrain compact.
Getting old is lived by the poet as a problem. Approaching to death simulate insecurity.
Going on reading, a comparison and another metaphor can be founded: boughs are compared with "bare ruined choirs" (v.3); the function of the metaphor, instead, is to describe the brunches as a person.
So, the first quatrain introduces the situation. While in the first one nothing personal is told (he only speaks about autumn), in the second one he uses the expression "in me" and speaks about the sunset. Growing old is the problem and is compared to the sunset (part before the night). This two extended metaphors fits perfectly with the Fair Youth's situation.
Few elements stress the presence of the death's semantic field: by and by, black night, to rest. Thus, the reader understands the process of death is felt profoundly.
Shakespeare created a cohesion between the second the and the third quatrain with an anaphora. All the third quatrain can be considered as metaphor, too. The rhyming couplet introduces a moral: "to love that well which thou must leave ere long". So, in order to not have regrets, you must appreciate things you have now, because one they they will surely disappear.
This poetry is full of other significant elements: semantic cohesion which bring us understanding that his life was going to finish; Shakespeare refers to visual elements, to the semantic fields of time and death. He also used words linked with the concept of fade: twilight, sunset, fadeth, west, glowing of fire, ashes, by and by, lie.
Concluding, the climax (which has been creating going on reading) is all concentrated on the word "deathbed".