The "Sara" here refers to Sara Fricker, to whom Coleridge was engaged when he started writing this poem, and whom he married while he was still finishing it. The fact that the poem is framed as an address to a specific person makes it one of Coleridge's "conversation poems." Scholars have identified six of Coleridge's poems as conversation poems because they share certain key characteristics. Most notably, they're all written as an address to a specific person. Sara's presence in the poem as the addressee will influence the direction of the speaker's thoughts. Her presence as the poet's fiancee/wife also suggests that love and marriage will be one of the themes of the poem.
This poem, like Coleridge's other conversation poems, is written in blank verse, a metrical form that uses unrhymed iambic pentameter. One line of iambic pentameter contains five iambs, or five pairs of unstressed-stressed syllables. The first line establishes this meter (though already there is some variation, with the double stress of "soft cheek" creating a spondee in the fourth foot):
My pen- | sive Sa- | ra! thy | soft cheek | reclined
The first line also establishes the poem's use of enjambment. In the first two lines, for instance, the speaker notes that Sara's cheek is "reclined," but the completion of that thought, that it is reclined "Thus on mine arm," doesn't come until line 2. Nearly all the lines in the poem are enjambed, as the speaker expresses himself in long sentences that flow over many poetic lines (the first sentence, for example, goes all the way from line 2 to line 10!).
These long sentences, with their many clauses, suggest that the speaker isn't planning out his words ahead of time, but instead speaking casually, expressing words as they come to him, just as people do in ordinary conversation. This establishes an intimate, informal relationship between the speaker and the addressee (and also might subtly reflect the way that poetic inspiration merely comes to the speaker passively, rather than being something he actively controls).
Besides establishing the addressee and the meter, the first lines also make the poem's setting clear. The conversation poems all begin by describing a vivid physical environment in detail, as the speaker here describes the color and shape of the flowers and the changing light in the clouds. The speaker reinforces the sense that this setting is quiet and peaceful through his use of consonance. The /s/ and /m/ sounds are repeated in "My pensive Sara!", "thy soft cheek," "Thus on mine arm," "most soothing sweet it is," and "sit beside." The fact that the repeated consonant sounds are soft ones contributes to the overall sense of euphony in these lines and in the whole stanza.
After describing the physical setting, the speaker in the conversation poems usually then moves into more abstract thoughts inspired by this setting. We see a hint of this pattern right away in lines 2-6. The speaker establishes that he and Sara are sitting "beside our Cot" (cottage), surrounded by particular varieties of flowers (jasmine and myrtle), but then notes that these flowers could be seen as symbols of more abstract ideas (innocence and love). This small movement from the concrete to the abstract will be replicated on a larger scale by the poem overall.
Finally, these opening lines suggest what key themes might emerge in the poem. The fact that the speaker takes elements from nature as symbols of larger ideas introduces the theme of nature as poetic inspiration. Because the flowers symbolize spiritual values, like love, there's also a suggestion that the natural world may be connected to the spiritual or divine world.