The basic unit of an iambic tetrameter poem is the iamb, a very specific kind of poetic rhythm. An iamb is two syllables of sound, the first syllable unstressed and the second one stressed. Iambic tetrameter consists of four iambs per line, so the rhythm of a poem written in iambic tetrameter sounds like "da DUM da DUM da DUM da DUM." You'll need to keep this rhythm in mind as you write your poem, because this particular metrical rhythm is the defining characteristic of iambic tetrameter.
Other Poetic Characteristics
Once you have the basic syllable pattern of iambic tetrameter down, the subject, tone, length and rhyme scheme (if any) of your poem are negotiable. Some poems written in iambic tetrameter deal with classical subjects and follow strict rhyming patterns. Others may use rhyme to achieve a comic effect. Still others may be completely without rhyme, or may address themselves to unexpected or surprising topics. Whatever your style or subject, as long as your poem follows the rule of having exactly four iambs to each line, you'll have a poem in iambic tetrameter.