To Autumn is one of the most esteemed poems in the language. I envy your opportunity to study it in a classroom setting! I can't explain the entire poem as well as some here, but here are a few things to look out for:
Boring detail: The apostrophe is often used to keep the pentameter intact. "o'er" is one syllable whereas "over" would destroy the meter. As for "moss'd" and "clamm'd" and "brimm'd" it doesn't seem necessary. Maybe (I'm out on a limb) he's distinguishing from "mossed" using two syllables, which he occasionally does, but then he uses an accent over the e. Look into this when you get a chance, especially if you don't understand meter yet.
In this poem, notice he doesn't use traditional fall imagery! Ask a class of 20 students to write an ode "to autumn" and they will return something with colored leaves, cold winds, etc. Keats is not being cliche, he's being inventive.
Notice how many times he vacillates between nearsighted imagery and farsighted imagery. Small things like slow oozing and gnats are described, but also distant things like the rosy sky and swallows. This helps create a complete picture, micro and macro.
Finally, just notice how dense his imagery is, and find words that evoke each of the senses. This wasn't really too common before the 19th century to be so relentlessly concrete. This is a very thick poem for being so short! There's much more to say, but maybe this can give you a start. Finally, many have attached philosophical significance to this Ode, and probably rightly so. Keep reading it!