The most important ingredient to writing an excellent poem is to write about a topic which moves you. Think about what you are passionate about whether that be a thriving marriage, a broken relationship, a social ill or an historical event. Select a topic which you have great first-hand or book knowledge about. If you love birds and birdwatching, write a poem about a specific bird that interests you. Think about the topic and make a list of words that describes the subject or your emotions about it.
One of the best ways to learn how to write excellent poetry is to read others' poems. Spend time each day reading the works of poets laureate. Research poets who've written about the subject you want to write a poem about and read what they have penned. Spend a few weeks just reading. Then, move to dissecting what those poets do that makes their work excellent. Identify poetic devices used such as alliteration, assonance, consonance, internal rhyme, imagery, metaphor or onomatopoeia. Identify the cadence of their work, whether its freeform or follows a precise structure, such as iambic pentameter or limerick.
Consider which structure and devices most resonate with you and practice recreating those devices. Think of poetry as a muscle that gets stronger with exercise. Practice writing simple poems using devices like rhyme and alliteration before beginning your poem. Select a time when you are unhurried, relaxed and inspired to draft it. Finish a first draft and take a break from it. Come back to it the next night or a few days later and read it through, making adjustments to words and lines. Repeat this process, spell-checking and reviewing your poem three times.
Improve your poem further by taking it to other poets to have them give you feedback on how you can improve your writing and imagery. Join a local poetry club or writer's organization. Reach out to professors in the creative writing or English departments at a local college or university and ask for feedback on your work. Gather feedback and consider it thoughtfully. Resist the urge to take criticism of your work personally. Instead, view your poem objectively, separating your emotions and self-worth from the writing. Apply the feedback you've gained to your poem, rewriting it as necessary. Finally, submit the final draft of your poem to poetry journals or local writing competitions for publication, if desired. You can also conduct a reading of your poem at local poetry reading nights in your area.