What devices of language—diction, imagery, figurative language, symbolism, irony, allusion— does Shakespeare use to achieve his effect in Macbeth? Explain how he utilizes each device (give examples from the poem and explain how each functions to communicate the theme of the poem).

Well, Macbeth is a full-length play, so it would not be possible for us to address the entire text, with respect to your query, in one response. Let us, therefore, examine the first scene. Shakespeare's three witches speak here in a meter called trochaic tetrameter, different from the way in which Macbeth and the other major characters will speak in iambic pentameter. The metrical foot of their speech is called a trochee (one stressed syllable followed by one unstressed), and there are four (tetra-) feet per line. For Macbeth and other nobles, the metrical foot of their speech is called an iamb (one unstressed syllable followed by one stressed), and there are five (penta-) feet per line. In the quotation below, I will place stressed syllables in bold font and divide feet from one another with a "|" symbol. The second witch says,

When the | hur ly | bur ly's | doneWhen the | bat tle's| lost and | won (1.1.3–4)

There are four feet per line, though the last foot in each line is truncated (the final unstressed syllable is dropped). The missing final foot actually lends the lines a sense of incompleteness, as though there is something more coming, as though something else lurks on the way. In addition, trochaic tetrameter sounds more menacing and aggressive than an iambic meter, where the first syllable of each line is unstressed. Further, the witches' speech here makes use of end rhyme—see how "done" and "won," the last words of the line, rhyme?—and this makes them seem more otherworldly. It sounds like they are casting a spell—which, in a way, they are. They are making their plans for the future, and their ability to rhyme like this almost makes it sound like they can predict the future.
Moreover, the sisters speak using paradoxes, saying,

Fair is foul, and foul is fair;Hover through the fog and filthy air. (1.1.12–13)

How can something be both fair and foul? It's either good or bad, right? How can something good be bad and something bad be good? We must wait to see how this unfolds and resolves, though it doesn't sound particularly positive in general. The use of paradox makes the sisters seem sneaky, deceptive, intentionally confusing. This effect also likely has to do with the alliteration in these lines. Alliteration refers to the repetition of the initial consonant sound in words that are near one another. Here, the "f" sound is repeated at the beginning of fair, foul, foul, fair, fog, and filthy. If you make the "f" sound, you'll hear that it's pretty breathy; in fact, "f" only makes a sound because we are pushing air out from between our top teeth and bottom lip.
A few things happen here: first, we have to bare our top teeth in order to make the sound, and so someone making the sound over and over would look like they're growling and snarling; second, it sounds close to the "s" sound so often associated with snakes and evil; third, the breathiness of the sound makes it seem misty and elusive, just like the sisters themselves. None of these clues, produced by the "f" alliteration, bode well for Macbeth, whose fate the witches discuss, and they all contribute to the dark and foreboding mood.