The speaker begins the poem by talking about her hair. (We're betting that she's a she here, at this point, since we have no evidence otherwise. Check out our "Speaker" section for more.)
So what does she mean when she says her hair had hardly covered her forehead? Well, it's probably a reference to the fact that the speaker of the poem is very young. She's so young, her bangs haven't even covered her forehead properly.
We also notice that the poem starts with the word "My." This is important because it puts emphasis on the speaker of the poem: she's at the center of this poem.
We'll also notice some alliteration in these first lines, with repetition of the letter H. Of course, given that this poem was originally written in Chinese, we don't know if that alliteration exists in the original language. But in the English translation, the repetition of the H sound here gives some rhythm to this first line. (Check out "Sound Check" for more on this poem's sounds.)
I was picking flowers, playing by my door, When you, my lover, on a bamboo horse, Came trotting in circles and throwing green plums.
What a cute scene the speaker paints for us here. She's a little kid playing by the door and her "lover," who is himself only a kid at this time, comes by. We'll notice that the speaker addresses her lover directly in these lines, saying "you." So this poem is addressed to the speaker's sweetheart.
In these lines the speaker also makes comedy out of romantic conventions. Instead of giving us a lover on a real horse (kind of a like a knight in shining armor), we get a little boy on a bamboo horse, a toy horse.
And what is this heroic lover doing on his bamboo horse? He's throwing green plums around. (Who knows why? It's fun to throw around green plums when we're a little kid, we guess.)
So these lines give us the beginning of the romance between the speaker and her lover. This is a romance that goes way back, to childhood.
We lived near together on a lane in Ch'ang-kan, Both of us young and happy-hearted.
Here the speaker lets us know that she and her lover grew up in the same 'hood: in Ch'ang-kan. This is an ancient city in China, which is referred to in the title of the poem as "Changgan."
By saying that they were both "young and happy-hearted," the speaker again puts emphasis on the idea of age: they're young. Not only that, they're happy—good times, gang.
Before we leave this first stanza behind, take another look. Notice any patterns of rhyme or rhythm? Yeah, us neither.