This poem is about just what it says in the title: first love. The first time the speaker ever felt that tug of his heartstrings is described completely in this poem. After he sees her for the first time and falls for her, he takes us through the different things that happen to him physically, including his hear, his appearance, his sight and his overall feelings.
"My face turned pale as deadly pale My legs refused to walk away" says that her face alone caused him to stop in his tracks and not be able to move. "And then my blood rushed to my face and took my eyesight quite away" shows how the color came quickly back into his face and he "blacked out" because of his emotional state. And the speaker ends the poem with "My heart has left its dwelling-place and can return no more." This just says that now that he has seen her, he will never be the same. She forever has his heart.
It's a straightforward poem. There really are no hidden meanings. It's all about that first love.