Now, it's a major rule in reading poetry not to confuse the speaker with the poet him- or herself. Often, a poet can use the first person ("I") to speak in a voice that is radically different from their own personal, biographical one. But rules, Shmoopers, were meant to be broken. So, we're going to say that the speaker of this poem is John Keats himself. It's not too tough to make this call, actually—the title tells us that this poem was written on a specific occasion in the poet's life.
It is important to remember the specific biographical context of the poem, though. This is written spontaneously, in one night, after "looking into" a translation of Homer. Keats is only twenty-one and has only recently become intoxicated with the idea of poetry. Before that, he was an apprentice to a surgeon (which was, needless to say, unpleasant in the early nineteenth century). Keats is ambitious, though, and desperately wants to do something great.
By focusing on this speaker, then, we can see in this poem some self-justification for a poet's life. He relates an experience of being blown away by poetry and wants to do that for others. We can also hear some of that youthful passion—he feels like an explorer standing on top of a mountain looking out at an undiscovered ocean.