Your first step in writing a critical appreciation of Shakespeare's Sonnet 18 ("Shall I compare thee to a summer's day?") should be to analyze its prosodic structure. You should note that it follows the standard form of the English sonnet, consisting of three open quatrains and a couplet, all using iambic pentameter. Despite being rhymed as an English sonnet, it has a classically Petrarchan turn after the second quatrain. You might wish to discuss how this generic echo of the Italian form makes this more effective as a sort of anti-Petrarchan sonnet. You might also wish to note that using an established genre for love poetry enhances the claim of eternal preservation of beauty in poetic form made in the couplet.
Your second task will be analysis of the way Shakespeare uses comparison. You should discuss how the poet starts by making us assume that the poem will be a fairly straightforwardly encomiastic work, praising the beloved as even better than a summer day. In fact, however, we discover that the poet is not so much praising his beloved as himself and the power of poetry to grant immortality to its subjects. Thus you might want to talk about the effectiveness of the way Shakespeare contrasts the perfection of art with the imperfection of material and ephemeral things.
First let me talk about its structure. Even if you are not a fan of poetry or Shakespeare, you can admire the craftsmanship with which he wrote his sonnets. Because it is a sonnet, it is a 14 line poem. That's mandatory. Depending on the sonnet type, rhyme scheme can vary, but a Shakespearean sonnet has the following rhyme scheme: ababcdcdefefgg. That scheme will break the poem into three quatrains followed by a couplet. The meter will be iambic pentameter. That means each line has 10 syllables. That's amazing to me that poets can do that. It's even more amazing that they can follow an unstressed/stressed pattern within each line. And rhyme!
The poem itself is about Shakespeare comparing his beloved to a summer's day. Well, actually he spends quite a bit of time writing about why summer isn't all that great. It's hot, it's windy, and it ends too quickly. He tells his beloved that she is much more tempered than the crazy summer. Shakespeare then goes on to write that even though summer will fade, his beloved's beauty will never fade like summer does. In fact, her memory and beauty will live on forever. At least as long as people can read anyway, since he wrote about it in this sonnet.
"So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee."