The entire poem's literal meaning is that because Langston Hughes is black, he cannot achieve his dreams. Racism, prejudice, limited opportunities and discrimination keep him back from achieving his dreams that he had when he was young. When he was young, he was optimistic and naive, and didn't know how cruel the world was, so he felt free to dream about achieving great things. But as he got older, his dreams were blocked by the world and its cruelties.
He describes this very literal reality through the use of figurative language. He uses a simile (comparing two things using like or as) to compare his dreams when he was younger to being "bright like a sun". He then uses a metaphor (comparing two things not using like or as) to compare racism, prejudice, discrimination, and all of the obstacles the world throws in his path to a "wall" that keeps rising until "it touched the sky"; so in other words, the world (a wall) blocked out his dream (the sun), making it almost impossible to achieve it. He compares the despair that this brings to "shadow" and that he "lie[s] down in the shadow," defeated. He continues all of those figurative symbols-the shadow, wall, sun, darkness, light-through the end of the poem as he commands his hands to break through the wall so that he can achieve his dreams. The only time the language is literal is when he declares, "I am black" and that he has "dark hands" which makes it all the more startling because of its blunt truthfulness, and all the more insulting that the only reason for this awful obstacle is his black skin.
As for the structure of the poem, try submitting another question on that tomorrow, as the format of this website allows for one question a day. I hope that the above descriptions of the literal and figurative meanings helped though! Good luck!