How am I meant to analyse an unseen poem when I can't even work out what the poem is about?

Firstly, don't forget that it's normal not to know what a poem is saying straight away! The whole idea of a poem is for a poet to put forward an idea or thought implicitly, using metaphors, rhyme, meter (and so on) to put the meaning across without being too blunt. To work out what a poem is about, it's helpful to try and paraphrase lines or sections of the poem - by writing what it's saying in your own words just next to the passage you're paraphrasing. Then, once you've gone through the poem doing this, look back over your paraphrased notes and see if you can spot any themes, recurrences, or differences within the poem - for example, a poem's first stanza might be describing a dark, stormy night, but the final stanza might describe a calm summer's day, and then you can begin to think about why this contrast occurs. If this still feels too difficult, or you've done this and you're still not sure where to go next, then try and forget about what the poem means for now! Focus instead on picking out isolated poetic techniques that you know and can spot - such as tricolons, metaphor, alliteration, rhyme... - and annotating your copy of the poem by circling or underlining them. This can similarly help you to spot any themes or patterns in the poem, and can also help to make the poem feel less cryptic and unknown. Every time you read and annotate the poem over, it'll become gradually more and more clear, and sometimes talking about the poem with someone else, or just starting to write about it, will make the poem more understandable.