According to Horace, poetry is a blend of reality and fantasy that both informs and entertains the reader. "He who picks his subject correctly will discover that neither words nor lucid arrangement fail him," Horace observes, because "sound judgment is the root and wellspring of good writing." In other words, you should pick a topic that is interesting and not already over-explored. Then use your understanding of how language works (grammar, diction) to create something new and exciting that expresses yourself and your subject clearly. To start with, you need a theme or central idea around which you can build a poem. This could be as simple as "love" or "death", but it could also be more specific such as "fatherhood". The only rule is that it must be relevant to you or your readers. There are many great poems out there that have never been written before or since because their authors were afraid to take a chance. They didn't know what would happen if they wrote about something that wasn't familiar or not directly related to them. But these poets still found ways to express themselves through their subjects enough to keep writing until they reached success. After you decide on a theme, the next step is to choose appropriate words and construct a sentence structure that will help you achieve your goal of informing and entertaining your reader at the same time.