Write a line of poetry. When you're finished, review what you just wrote and look for a pattern of stressed syllables in the words you chose. A stressed syllable is the part of a word that you put emphasis on when you say it aloud. Quieter syllables are termed unstressed. For example, in the word "em-pha-sis" the syllable pattern goes, stressed, unstressed, stressed.
Find the dominant meter, or pattern of stressed and unstressed syllables, in your line of poetry. The meter could be iambic, where syllables are paired in an unstressed-stressed pattern, as in the word "a-way". The meter could be trochaic, where syllables are paired in a stressed-unstressed pattern, as in "o-ver". Alternatively the meter could be dactylic, where syllables are grouped into three: stressed-unstressed-unstressed, as in "gen-tl-y". The meter could also be anapestic with syllables grouped into three: unstressed-unstressed-stressed, as in "cav-a-lier" for example.
Choose the meter you want to write your poem in and revise your first line to run according to that poetic meter by changing some words so that they follow right pattern of stresses. Begin and end every line with the proper sequence of stressed and unstressed syllables and use the same number of syllables per line of poetry.