With how sad steps, O Moon, thou climb'st the skies!
The speaker is clearly looking at the moon to start this poem. He uses an apostrophe ("O Moon") to address the Moon and to comment on how sad the steps are that he's using to climb the sky.
Okay, so let's slow down just a second. Sad steps? Climbing? What's the deal here? Are these the "steps" of some crazy ladder in the sky that the Moon is climbing?
Eh, no (though that's a great guess). Actually, the speaker is personifying the Moon—pretending like it's a person who can actually "climb" things. Hmm.
You know how the sun rises? Well, the moon does the same thing. That's what's going on here.
So, what makes these steps sad anyway? The short answer is: probably nothing. However, we're gonna guess that the speaker, who is one love-sick dude, is doing some projecting here. He's a little sad, and he's seeing images of sadness everywhere around him.
The Moon is probably creeping up the sky really, really slowly, which makes the speaker personify it (the Moon) as some kind of sulky, shuffling planetary body.
Before we continue, we've gotta tell you a little something about the word "climb'st." A letter is missing ("e"), which means you should pronounce this guy as one syllable—not "climb-est" but "climbst."
You've probably already guess'd by now (see what we did there?), but "climb'st" just means "climb." In the old days, people used to end words with endings of "-est" or "-st" ("climbest," "feelest").
If the speaker had used the "-est" version, the result would have been a two-syllable word ("climb-est"). Here he opts for the one-syllable version in order to make the meter (which in this case is iambic pentameter) work. (Two syllables would have given the line eleven syllables—one too many.)
The speaker continues to address the Moon (even though he doesn't actually apostrophize it directly again).
Here, he comments on how "silently" and with "how wan a face" the Moon is. Cheer up, Moonie.
The word "wan" means lots of things—none of them good. It can mean pale, pallid, or sickly.
The moon doesn't just look white. It looks pale and sickly, kind of like Gollum.
Notice that the word "how" has already appeared three times in this poem. (If we're lucky, maybe we'll see it again.) The speaker keeps using it because he's trying to express surprise or exclamation, like when we say "OMG, how handsome you look," except in this case it's more like "OMG, how sad and sick and silent you are." Yeah, not so good.
The moon is usually white, so it's possible that the speaker is not only personifying the moon again (the moon doesn't really have a face), but projecting his own feelings onto it. He probably feels a little wan himself and is seeing versions of himself all over the place.
What, may it be that even in heavenly place That busy archer his sharp arrows tries?
Since the speaker has managed to convince himself that the moon is "wan" and sad, he tries to figure out why.
He ponders whether "that busy archer," Cupid (more on him in just a sec), is also shooting arrows up in the "heavenly place" where the moon resides. "Heavenly" just means the "heavens," or the sky.
In other words, the speaker wonders whether the moon is having some lady problems. That's the usual way of thinking about Cupid (a.k.a. "Eros"): he shoots or tries out his arrows on people and makes them fall in love (this is a very old idea, by the way—check it out).
His arrows are "sharp," which means it probably hurts to get shot with one—duh. More importantly, this is the speaker's metaphorical way of describing the pains of love: it's like getting shot with a "sharp" arrow by this little dude.
(Cupid, as you may know, is a major figure in Greek and Roman mythology. There are tons of stories about him. To get a feel for some of his extracurricular activities, read about his involvement with Psyche right here.)
Did you notice that the verb ("tries") comes at the very end of the sentence? In normal speech, we would say "what, is it possible that Cupid tries his sharp arrows?" Poets do this stuff all the time.
Why? Well, because they can, for one thing. Also, the line now make rhymes with line 1 ("skies"), which it couldn't do if it ended differently.
As for the poem's rhyme scheme, at this point it looks like it's going to be something like this: ABBA. That's the scheme for the first four lines at least. We'll keep you posted.
It looks kind of like a sandwich (the A's are the bread, the B's your peanut butter and jelly). Head over to "Form and Meter" for more on this, and an explanation of why this funky sandwich rhyme scheme is important (and also awesome).