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?
Let's tackle what's going on in that order.
Singing a country. 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. We won't do all the work here, but instead ask you to mull it over: how is speaking different from singing? And what effect does the phrase "sing America" have when contrasted with, say, "speak America"?
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. (Read Whitman's poem here.)
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.
So 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. He's also singing this song of America.
We'll also point out that one of Whitman's poetic trademarks, as it were, is a forceful and ever-present sense of self – the "I" in Whitman's poems is pretty much constantly in your face. (Want an example? Check out "Song of Myself.") There's a similar sense of self here in Hughes's poem too, too – after all, the first word in the poem is "I!"