How is the best way to compare themes in two different poems?

First of all, you need to recognise what the key themes are of the poems in isolation. For example, take the first poem and see if you can recognise any recurring imagery, like the image of a moon in the first and last stanzas. Go through the whole poem and make a list of the themes you can pick out.

Then, do the same with the second poem and see if any common themes crop up. If there is, then you can make an easy comparison by describing the themes and comparing the effect that they have on the different poems. There are always things that poems have in common and even if they don't you can compare the differences so there is always something you can say.