Whenever you have a question that asks you to summarize something, the best way to do it is to read it all through in its entirety, and then try to sum up what you read in your own words. Summaries tend to be about 1/3 the length of the original work; so, if you were reading 3 paragraph essay, your summary, written in your own words, would typically be around a paragraph. You want to summarize the main points, the main ideas, and the general gist of things.
To summarize a poem stanza by stanza, I suggest you read the entire poem through once so that you get the overall picture. Then, go back and write a concise summary of each stanza. You are in luck with Emily Dickinson, because her stanzas are pretty short, and her poems tend to be quite short also, so the summary won't be very long. I tried finding the poem "Trees" by her in order to help you summarize it, but I think that you must have the title wrong, because I can't find it anywhere (I have her complete poems). But to give you an example from a different poem of hers, in her poem "'Hope' is the thing with feathers," the first stanza reads,
"'Hope' is the thing with feathers--/that perches in the soul--/that sings the tune without the words--/that never stops--at all--"
If I were to summarize this stanza, I might write, Emily Dickinson, in her first stanza, compares hope to a bird sitting in the soul that sings no matter what.
I hope that these thoughts help a bit; if you can find the proper title to the poem you need help with (her titles are usually the entire first line of the poem), I can help you more specifically. Good luck!