How do I decide who the speaker?

In a poem, the speaker is the "main character" of the poem - it may be an observer or participant; many times (most?), it is not the poet just as a novelist is not the character in the book.

In Carl Sandburg's "Grass," the speaker is the grass - "I am the grass. Let me work."

I am not sure of the second part - as asked, I would guess the situation is what's going on in the poem, and the purpose is to capture a moment or pass along a message (theme).