I will help you by giving you a few "hooks" on which to base your understanding of this excellent poem. Hopefully, after reading my response you will be able to revisit the poem and understand a lot more. To start off with, let us look at a summary of the poem.
As the poem begins, we are presented with the speaker of this narrative poem, who is a weary student. He is studying at midnight and also mourning his dead love, Lenore. He hears a faint knock, and opening the shutter, he finds a mysterious raven. The talking bird amuses the speaker at first, but its refrain of "Nevermore" in answer to the speaker's pleading questions about meeting Lenore after death, drives him to despair and madness. As the poem closes, the bird settles in to stay, a brooding symbol and symptom of the speaker's desperate state of mind.
What is key to what this poem is about is the major theme of the poem. Poe himself said this poem explored one aspect of the dark side of human nature - to quote Poe, "that species of despair which delights in self-torture". We are presented with the figure of a troubled individual who is desperately mourning his lost love, Lenore. When the Raven enters he asks questions of it about Lenore, and is pushed into ever greater despair by the single word response, "Nevermore", which is placed in direct conflict with "Lenore", both in rhyme and in position. Of course, in the jargon of psychology, the narrator projects or puts onto the bird whatever his own wild imagination dredges up and thus tortures himself in his depressed state.
This is to say that the Raven of course is just a raven rather than a messenger from hell, but the raven suits the speaker's mournful tone and his tormented projections, and thus becomes a symbol of death, loss, or despair.
Hope this helps! Now go back and read the poem and enjoy.