An epigraph is a short quotation placed at the start of a poem to help set the tone and focus your efforts. Almost anything can work if you find it inspiring: a quote from another poet, a few sentences from a news article, a memorable phrase spoken by a friend, a saying from Poor Richard's Almanack.
A metaphor makes a comparison, and in doing so shapes our perception. If we say, "Time is a river," we're noting a certain similarity between the two. Yet we know they aren't identical. We may mean that time is fluid, has currents and eddies, empties into some vast ocean, but not that it's composed of water. If we say, "Time is a stone," we may mean it's silent, still, indifferent, but not that it's a mineral.
Pick a spot where you can write for a while without being disturbed. this could be a private spot where you are alone, or a public spot such as a coffee house or a park.
Begin by focusing on your immediate environment. Note the sights, sounds, smells all around you and start writing them down. As you do, let yourself get lost in your surroundings. You may want to to use apostrophe, or to shift perspectives.
After four or five minutes, turn your attention gradually inward to your experience of the scene—what it reminds you of or how it makes you feel, for instance. Don't try to control or direct this process, just tap into your internal language. And keep writing.