Sonnet 138 is not exactly filled with traditionally poetic devices such as metaphors or imagery. It is a more direct statement of the situation between the speaker and his lady. The central point of the sonnet is that they both lie to each other: he lies that be believes her to be a chaste maid, and she lies that she believes him to be a young man. It is a kind of mutual deception that connects them to each other. The one clever use of language is the pun on the word "lie." In line 13 the speaker says "I lie with her, and she with me," but this can be taken to means that they both tell lies to each other and that they lie together in a physical sexual way as lovers. They are happy together because they both accept the other as they actually are. This poem comes late in the sequence and is likely to be talking about the "Dark Lady" -- a lover that is referenced in several of the sonnets.
As the first commenter suggested, there isn't as much figurative language in this sonnet as might be in some others. However, in addition to the pun he or she identified in the word "lie" in line 13, the word "habit" in line 12 is another example of a pun. Here, "love's best habit" can refer to both a habit as a pattern of behavior as well as a habit as something that someone would wear, conveying the idea that love is best clothed, so to speak, in "seeming trust" (line 12). This denotation could be interpreted as comical as a result of the narrator's complicity with the idea of being dishonest in love. However, both denotations are equally appropriate in this context.
Additionally, you could make the case that line 1, "When my love swears that she is made of truth," the lover (the speaker's mistress) is employing hyperbole, or exaggeration, in order to convince the speaker of her veracity. She isn't actually "made" or composed of truth, as though truth were a tangible substance; she is merely insisting that she is honest.