The content of this poem is hard to wrap your mind around, so fortunately the form is standard English Renaissance stuff. The poem consists of five rhyming couplets: yesterday/away, past/last, two/do, you/too and I/die. You can see that lines 5-6 also rhyme with lines 7-8, but they are still considered couplets. Also, be careful: this one is only ten lines long, so it's not a sonnet. Sorry, sonnet lovers. You'll have to check out some of Donne's other amazing work, like Holy Sonnet 7, Holy Sonnet 10, or Holy Sonnet 14 to find your fourteen-lined friend.
"The Computation" begins with an interesting rhythm:
For my first twen-ty years
These first six syllables contain two anapests. An anapest is two short beats followed by a long, accented beat (da-da-DUM).
As for the remainder of the poem, the meter is all over the place, but the closest we can approximate is iambic pentameter, which is a ten-syllable line with five iambs, or short beats followed by long ones (da-DUM). Here's a perfect iambic pentameter line:
For for-ty more I fed on fa-vours past
But Donne bends the normal rules of time out of shape in this poem, so why shouldn't he bend the rules of meter, too?