And let that page come out of you—Then, it will be true. (4-5)
This, the teacher's assignment, sets up the whole poem to be about finding the truth in one's own self. If something comes out of you—is truly part of your identity—then it will be "true," this assignment posits. Now of course someone could sit there writing "blah, blah, blah," and that probably would come out of them, but it wouldn't have much insight or truth in it. So it's up to our speaker to determine how successful the speaker's assignment will be for him.
I am twenty-two, colored, born in Winston-Salem. (7)
This is the first line in which the speaker is writing about himself. So it's likely the most basic, integral part of his identity. He's young, black, and born in the south. Given the date this poem was published—1951—we know that it's likely that, as the speaker guessed in line 6, the assignment would not be as simple for him as for a white person his same age.
[…] But I guess I'm what I feel and see and hear, Harlem, I hear you:hear you, hear me--we two--you, me, talk on this page. (17-19)
Though we know that our speaker was born and brought up in the South, here, we hear about how living in Harlem affects his identity. He feels as if he converses with the city—there's plenty to see and hear and feel in New York. But it's not one-way. As he's affected by the city, the city is affected by him. His experience of Harlem is different from the experience that anyone else could have in Harlem. And that's part of his identity.