Well, the title and the first line are the same. So you can refer to our "Line by Line Summary" to get a grip on what that first line is all about – that's a good start. But why make the first line the title, in the first place?
Sometimes, the very first line of a poem does everything that a title should do – which is sort of a ridiculous thing to say, as titles are capable of doing a great many things. In the case of "I, Too, Sing America," that first line introduces us to the patriotic theme that will show up throughout the whole poem. It also brings to mind Whitman's "I Hear America Singing."
If you haven't read it, Whitman's poem 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. Hughes seems to be referencing Whitman's poem and improving it by adding another voice to this song of America – the voice of black Americans.
So if you've got all that going on in the first line of your poem, you don't really need a different title, now do you? (No, really – what do you think?)