This poem can pretty tough slogging, mostly because of the compound nouns Dickinson uses. If you find yourself struggling, just know that almost everyone feels that way, even literary scholars! In fact, the only way that we "know" that the poem is about a hummingbird is because the poem can be found in one of Dickinson's letters, where she states that she is writing about a hummingbird.
Remember, Dickinson had no intention of anyone reading this other than the letter's addressee. Dickinson left instructions for her sister to burn all of her poems, so she wasn't necessarily writing with a wider audience in mind. If you go into the poem expecting to see a single, clear image, you'll just end up frustrated. Instead, take the poem in chunks, and deal with a few words at a time. Instead of a concrete whole you'll end up with disparate pieces, and we think you've done a good job if you only end up mildly confused. After all, the poem is about being confused—but also amazed—by something.