Rhyming is easy; doing it well is not so easy. Rhyming lines can easily turn into sing-song verse that lacks depth and metaphor and relies only on rhyme and narrative, rather than symbol and image. To write a rhymed poem, it's best to resist the urge to preconceive the poem, which will limit your choices and your subconscious imagination. Instead, let it grow organically by working with the language rather than an idea.
Decide on a pattern of rhyme, which is what poets refer to as a rhyme scheme. In a rhyme scheme, the lines are labeled with letters: A, B, C and so on. "A" lines rhyme with "A" lines, "B" lines rhyme with "B" lines and so forth.
For example, you might rhyme every other line, which is an ABAB rhyme scheme. Or you might use ABBA, which means that you will rhyme the first line with the fourth line and the second and third lines with each other. From there you can use a CD scheme, such as CDCD or CDDC in which you use the same pattern but with different rhymes. The poet leaves white space between each "stanza" -- a set of lines that work as a unit within a poem.
Write a first line that works meaningfully. It needs contain a subject and a verb, and ideally, it will resonate, or strike a chord in the reader. To do this, use a strong verb. For example, use "stumble," "trudge" or "trip" instead of "walk." Try to end the line with a noun or verb. Also, keep in mind that prepositions and conjunctions make particularly weak line-ends.
Make a list of words that rhyme with the last word of the first line. Omit nothing, even if it seems silly. Use your imagination or a rhyming dictionary. Keep in mind that a final syllable may make a good rhyme with a single syllable word. For example, "rage" rhymes with "language."
Write a second line. You don't need to worry about finding a rhyme yet, unless you plan to work with rhyming couplets -- two-line stanzas that work as a unit and rhyme with each other.
Write a third line either to rhyme with the first line or the second line, depending on your rhyme scheme.
Continue writing lines, rhyming them according to the rhyme scheme you chose. Aim to write lines that rhyme first, then revise the lines to make grammatical sense if necessary. Keep in mind that the end of the line does not have to be the end of a sentence. You can "enjamb" (pronounced "en-zhahm") a line, which means that the sense of it does not coincide with the line end, but instead continues on to the next line. For example:
Here, he trips through the snow and fields
of his boyhood. Here, at the brook, he once
fell through the ice, silly dunce
that he was, and here where the snow yields
to spring, and the green grass now grows in the field,
he wants to follow the old tractor's ruts
home to his sorrowing mother, whose heart he once cut.
Notice that this example contains only two sentences, but the poet has enjambed them. Also notice that this passage has an ABBA rhyme scheme.
Break the poem into stanzas if suitable. You might have stanzas of four lines each, or you might use couplets. You can also use irregular stanza lengths. You can do whatever you want if you are crafting your own form -- many contemporary poets do.