When you are analysing an unseen poem, it can be a little more difficult because you may not be aware of the context surrounting the text, and so it might not be immediately obvious what the poem is about. Try not to panic - interpret what you think the meaning of the poem might be as best you can. Keep this idea in your head and relate the points you make in your writing back to this meaning as much as you can. This is the best way to make a clear argument. Always remember to comment on the form of the poem; its rhyme and metre if possible; the tone of the poem - is it melancholy, hopeful, inspirational, introspective?; and pick out plenty of poetic devices such as alliteration, onomatopoeia, and so on.
The most important thing to remember is once you have picked out these features, you must talk about why the poet might have chosen to use them. As long as you make your argument clear and you are able to back up your interpretation with quotations, there are no wrong answers, so don't let the fact that the poem is unseen throw you off.