Context

Right off the bat, this poem opens up some questions. 

For one, how can someone "sing" a country? And, secondly, who is the speaker referring to when he says "too"? Who else is singing America?

Singing is a particular kind of speaking, so maybe to "sing America" means to tell someone something about America, or to speak of America. (Note, also, that poetry is intimately connected with music, so the verb "sing" works really well here.) And even more on singing: think about what it can do that normal speaking, really, cannot. It can highlight emotion in different ways and can call attention to different things. 

As far as the "too" is concerned, look no farther than Walt Whitman's "I Hear America Singing," the poem which some scholars think inspired Hughes's poem. In Whitman's poem, he lists all sorts of different Americans—carpenters, mechanics, boatmen, shoemakers, a girl sewing—and says that all of them are singing. We get the picture that America is like a song made up of many different voices singing. So Americans are a kind of chorus, where every person has an important part to sing. Maybe Langston Hughes's speaker is imagining Americans as a big chorus, all singing together, and he's saying he's part of the chorus...too?

Where you've heard it

This phrase—"I, Too, Sing America"—has been adopted by lots of people for lots of reasons:

• It was the name of an tribute to victims of police brutality.
• It was the name of an article by a history teacher.
• It was the name of an essay by author Julia Alvarez.
• There was an "I, Too, Am Harvard" performance that led to a cross-campus campaign.

Pretentious Factor

If you were to drop this quote at a dinner party, would you get an in-unison "awww" or would everyone roll their eyes and never invite you back? Here it is, on a scale of 1-10.

There's nothing pretentious about singing America.