First of all, if you do not have a subject for your poem, you will need to brainstorm about what you will write. Try to avoid the usual topics of love or friendship; perhaps you could write about some aspect of a day such as dusk or dawn or about a creature of nature as Emily Dickinson and D. H. Lawrence did. Also expressing your unique point of view about a feeling can provide ideas, as well. Miss Dickinson, for instance, wrote about hope being "a thing with feathers." When you decide upon a topic for your poem, then determine what the controlling metaphor will be. That is, to what are you going to compare this idea or feeling? The metaphor that you use should be a fresh and unusual one.
One you decide upon the subject matter of your poem, write lines that express the comparison of the controlling metaphor. For them to be iambic pentameter, there must be 5 feet (5 sets) of an unstressed syllable followed by a stressed one. The line's rhythm then reads ta dum, ta dum, ta dum, ta dum, ta dum. For examples of iambic pentameter refer to Shakespeare's Romeo and Juliet which is written almost entirely in iambic pentameter (or his sonnets). Of course, assonance is the repetition of vowel sounds and alliteration is the repetition of initial cosonant sounds. For instance, these two lines have alliteration of the /s/:
I craved strong sweets, but those
Seemed strong when I was young
And here is an example of assonance in which the a sounds contribute to the poem's musicality:
Cooked on the homeblack of a flat stove.
Flipped to slap the birth awake.
Wrapped by corn hands.
Take a walk and think; you will be surprised what good ideas spring from a good walk. Below is a site that the poets e.e.cummings advice to young poets "APoet's Advice".