Writing an essay which discusses the themes in a poem is pretty straightforward but first you need a poem. I guess you have been asked to choose one which has themes of death and time as its subjects. If not, it might be a good idea to post again, giving the title of a particular poem.
In general, however, it is customary to start a poetry analysis with the poem details (title, author, date, chronology of writing etc.) and a brief line about the context and main subject of the poem. Then go through the poem and write a draft essay which notes major ideas which occur again and again and which seem to preoccupy the poet. Take quotes when you notice these and use your knowledge of the poet's life to illustrate your opinions supported by the quotes. It is acceptable to give your personal take on it, provided you have shown that you know something about the poet's life and can support your theories with examples from that and from the poem ie quotes.