Here's the set-up (all of this is proven later): The narrator has committed murder. He might have done this out of malice, or self-defense, or anything in between; we don't know. The fact is that he killed someone, was caught and sentenced, and is now on Death Row. The man is not an important person, so to speak. He is not famous, nor rich, nor anything of the kind. He has no high-priced lawyers and no "connections" to help him in his plight. The narrator implies that, if he had higher social status, if he had money or fame or whatever, then he would stand a good chance of escaping death. But alas, he is merely a "poor boy" (aka ordinary person), and has no such power. His family and friends are attending the execution (or have otherwise heard about it), and are very distressed. Conversely, the family and friends of the dead man want revenge and they can't wait to see the narrator executed. The song takes place just prior to the execution, and involves the narrator talking to (or perhaps just thinking about) his mother, just before he dies.
If you're still reading, you have my thanks.
Here's the line-by-line analysis:
*We start with the narrator's thoughts:
"Is this the real life? Is this just fantasy?"
*The narrator is overwhelmed by the idea that he's going to die. He almost wonders whether this is all a nightmare or something.
"Caught in a landslide, No escape from reality"
Again, he feels overwhelmed, but he can't really deny that he's about to be killed.
"Open your eyes, Look up to the skies and see,"
Looking up to heaven, wondering about life etc.
"I'm just a poor boy, I need no sympathy
Because I'm easy come, easy go, Little high, little low"
Here he is quoting the common perspective: he's just a poor man ("boy"), and he doesn't deserve sympathy. Much of the song is about how no one seems to care for the narrator, even though he seems mournful and regretful for his actions.
"Any way the wind blows doesn't really matter to me, to me"
Now that he's going to die, nothing more matters. He has no future, no hopes or dreams or goals. He's going to die within the hour, and there's nothing he can do about it. He feels very hopeless, and from his perspective nothing really matters.
"Mama, just killed a man, Put a gun against his head
Pulled my trigger, now he's dead"
This part is obvious. He committed murder. I don't he's confessing to his mother here, as surely she would already know by the time of the execution. I think that he's really just sadly reflecting on what he's done, and he mentions this to his mother (or perhaps he's just thinking about her)
"Mama, life had just begun
But now I've gone and thrown it all away"
He was a young man, in his 20s perhaps. He had the chance to live a meaningful life, but instead he killed a man, thus causing his own death via execution. The narrator laments, noting that he could have saved his own life by choosing not to murder. But now the deed is done, and the narrator will face justice.
"Mama, ooh, Didn't mean to make you cry
If I'm not back again this time tomorrow
carry on, carry on as if nothing really matters"
Again he's sorry for his actions, and regrets that his mother now weeps for him, as he will soon be killed. The execution will take place within the hour, so if he's not back again this time tomorrow, it will mean that the execution happened on schedule, that he failed to escape it via pardon or other means. The narrator tells his mother that, even if he dies, she should carry on living, almost as if his death didn't matter to her.
"Too late, my time has come"
The execution is imminent.
"Sends shivers down my spine, body's aching all the time"
These are symptoms of his intense fear....