In literature, tone refers to the way the author feels about the subject of the work.  In this particular poem, Poe seems to sympathize with the narrator.  The speaker has endured a terrible loss—the death of his love, Lenore—and his perception of the world around him is quite tainted by his sorrow.  It wasn't just a cold December, but a "bleak" one; the embers in the fireplace are "dying" as the narrator tries to find some solace or distraction from his feelings of absolute loss.  In fact, he is so desperate for "respite and nepenthe"—a break from his sorrow and despair—that he first imagines the raven to be sent by God to distract him.  

Finally, the raven seems to take on much deeper symbolism when the narrator describes the bird's "shadow on the floor," a shadow from which his soul "Shall be lifted—nevermore!"  In other words, the narrator will continue to experience his loss of Lenore forever and ever; his sadness will never leave him, and he will never be comforted.  This is sad, indeed.  In this way, Poe seems to feel and create sympathy for the narrator by conveying the idea that the pain caused by the loss of a loved one can never be blunted or fade.  

Many teachers, critics, and students have their ideas of "The Raven," especially since it's such a widely read and popular poem.  Even if reading it at the college level, though, don't be intimidated by its popularity or by the overwhelming amount of literary criticism you may encounter concerning "The Raven."

At its core, "The Raven" is a poem about the narrator's lost love, Lenore, and his final realization that not only has the love of his life died, but he will never ("nevermore") be the same.  Like any good poem, the tone changes as the poem progresses.  Though many critics would say the overall tone is "melancholy," step-by-step, the tone progresses from distracting and distracted (the narrator delves into his studies as a means of distraction, and then a sound suddenly distracts him) to curious (exploring the sound), annoyed and angry (at the bird's insistence), then finally, resigned (to the fact that Lenore is gone, never to return, and that he will never get over her).  In a way, the narrator progresses through part of the stages of grief (shock and denial; pain and guilt; anger and bargaining; depression, reflection, and loneliness...though stages five through seven never seem to happen).

I would say that overall, the tone is resigned/despairing, which connects to the central idea that the narrator has realized his life will never be the same without Lenore.  Just remember that the message (central idea) and the tone are always connected.  Determining the message, speaker, and audience of a piece can help you determine the tone as well.

I hope that this information was helpful to you!  Let me know if you have any further questions!