You must let the words come, and try not to substitute words that rhyme for better word choices. Some poets use a rhyming dictionary to help them. Other writers just use a good Thesaurus. I prefer Roget's Thesaurus. There are many different forms that poetry can be written in, but I just covered the most popular types here.
Iambic pentameter is one of the most popular types of meter.
This is a poem that uses ten syllable lines in stanzas of four usually. A form of poetry that is similar to this is a sonnet. Sonnets have a specific rhyme pattern, though. A Shakespearean Sonnet is a structured poem consisting of fourteen lines arranged in three quatrains that are four-line stanzas, ending with a rhyming couplet. Each line of the sonnet is composed in iambic pentameter.