To find stressed and unstressed syllables, you have to listen carefully to yourself as you read the verses, preferably aloud. Which syllables, not words, do you emphasize? For instance, consider your own name. Let's say it's Tyler. Do you pronounce it TY-ler or Ty-LER? Or look at another word like eagle; do you say EA-gle or ea-GLE? The goal is to read a line as normally as possible.

Mark the stressed syllables. Usually a foot (or unit) of poetry has one stressed syllable; the number of unstressed syllables typically is no more than two. The next step is to divide the verse into feet, making sure you have at least one stressed syllable in each foot.

Here's a line from "The Eagle":

The WRIN|-kled SEA |be-NEATH |him CRAWLS = 4 stressed syllables. I can divide this line into four feet.

The pattern seen here is a foot of unstressed-stressed (u /) called iambic meter, the one most often used in English language poetry because it most closely resembles the way we talk.  

The number of feet here is four; the poetic term for that number is tetrameter. So this line is iambic tetrameter.

Like most poems, however, this one isn't written in perfect meter. The 2nd and 3rd lines begin with a stressed syllable; in fact, the first foot is the opposite of iambic; this pattern (/ u) is called trochaic.  Nevertheless, most of the poem is written in iambic, and the lines still have four stressed syllables. So we can say the poem is mostly iambic tetrameter.