One thing to keep in mind when writing a memorial poem for your father is you can find inspiration by gathering ideas through reflection on memories, looking through his belongings and reading letters or messages from him. Take notes and craft an outline with all your initial observations.
Find more inspiration by talking to people who knew your father or asking some important questions about your father's life. Answering questions about what your father's legacy is, how he would hope to be remembered or even what kind of words best describe the life he lived are a good way to find the language of the poem. Remembering his funny quirks, his favorite cologne or scent, a song he loved, or any acts of kindness he performed are other ways to bring the poem to life.
Once you have put together an outline or a first draft, it is time to decide what tone or mood to use in the poem. If the poem is solemn, go through the draft and eliminate any information or anecdotes that do not fit the tone. If the poem is funny or conversational, find the information that supports that mood. Once the ideas have been paired down, write or edit the poem and structure it in such a way that shares your heart and honors your father. Use other memorial poems as a reference, but remember that the poem does not need to follow a specific structure, length or rhyme.