A metaphor is
a figure of speech in which a word or phrase is applied to a person, idea, or object to which it is not literally applicable. It is an implied analogy or unstated comparison which imaginatively identifies one thing with another.
So, your first task is to decide what type of comparison you would like to make in your poem. It needs to be something that makes sense but is not directly obvious - the implication is what will make your poem a metaphor!
At the beginning of a new year, the picture of a baby is often used as a metaphor for the young year. You need to think of a word picture that can be presented as a comparison with something else. For an extended metaphor, you probably want to find an object that you can describe very descriptively as you point out many parallels between it and the thing you are directly picturing.
Can you create metaphors to describe an old junker of a car? If you had a whole colony of mice living in the remains, perhaps the music from the radio is the squeeky noises of the baby mice. The steering column guides the travels of the mice deep into the car's engine, where they run (like the car once did) and move like fire from one spark plug to another.