I need help writing a short essay about Walt Whitman's "Crossing Brooklyn Ferry"?

I appreciate that it can seem daunting knowing where to start with a poem like this. It's long, certainly, but it does have a unifying theme, which is important for focusing your essay. Rather than thinking about writing "an essay" in general terms on the poem, I would recommend you begin by narrowing that down. Look at this couplet, which, I feel, summarizes the theme of the poem:

What is it then between us? What is the count of the scores or hundreds of years between us?
Whatever it is, it avails not—distance avails not, and place avails not, I too lived, Brooklyn of ample hills was mine

This comes from stanza 5 of the poem. In it, Whitman challenges the reader: what is it that connects Whitman, as he crosses Brooklyn Ferry, to the scores of people who have done so before him, and who will do so after him? So, in writing on this poem, let's consider how Whitman presents the idea of being unified, through the places we know and regularly travel through, with people who are part of the same crowd, both past and future.
What devices and language techniques does Whitman use to convey this idea? Repetition is something he uses throughout the poem to emphasize the sameness of repeating a journey over and over, the sameness of all our lived experiences:

Just as you feel when you look on the river and sky, so I felt, Just as any of you is one of a living crowd, I was one of a crowd, Just as you are refresh’d by the gladness of the river and the bright flow, I was refresh’d,

In the use of parallelism here, Whitman draws an equivalent parallel between his experience and that of others. He also enumerates the things he remembers about, and associates with, the experience of crossing Brooklyn Ferry, using vivid imagery to make the experience vivid. Here, he presents images which would also have been experienced, at different times, by others sharing the same journey:

Saw how the glistening yellow lit up parts of their bodies and left the rest in strong shadow, Saw the slow-wheeling circles and the gradual edging toward the south,

Whitman uses this technique throughout the lengthy third stanza. You could also consider why this stanza is so long—why does he want to give us a vivid understanding of what it is like to cross Brooklyn Ferry?
Over and over, Whitman returns to this idea of a connection between those "years hence" and those he sees today. He also repeats the idea that we are at the same time part of something, and yet detached from each other:

The simple, compact, well-join’d scheme, myself disintegrated, every one disintegrated yet part of the scheme,

The repetition of the word "Others," as Whitman describes how "Others will watch the run of the flood-tide," "Others will see the shipping," seems to set these "others" apart. Whitman is literally "othering" these people whom he does not know. The point Whitman seems to be making, however, is that we do not need to know each other in order to be connected. Rather, so long as we know the same sights and experiences—"enjoy the sunset"—there is a connection between us: "I am with you."
Note also Whitman's direct appeals to the audience throughout the poem. He writes in the first person ("I") and addresses the reader as "you." This helps to convey Whitman's key theme of connection between himself and the Others, wherever they may be, and in what time. Another technique Whitman uses to create this intimacy is that of the rhetorical question, drawing the reader into his thought process:

We understand then do we not? What I promis’d without mentioning it, have you not accepted?

I hope this gives you some idea of where to start and what to focus on.