Up until yesterday, there was a guy in my cell who thought he was Jesus. Now, when you live in a prison, you’re going to find a lot of really strange people. Once there was a guy across the way who thought he was the Green Lantern. No joke. He even had a cheap, old ring on his right hand, the kind you get for a quarter in little plastic bubbles from the machine sandwiched between the two others that sell fake tattoos. The ring was harmless, or so the guards must have thought, so they let him keep it. They probably thought it was hilarious. It stopped being hilarious when they found him one morning dead, apparently having choked on his ring. The death was ruled a suicide, and was quickly forgotten.
But I’m already off topic. A guard eventually told me that my

I don't like that it seems to meander. We're told that we're going to get to a guy who thinks he's Jesus (and something happened to him); then we get the guy with the ring; then we get a comment that narrator is rambling . . . I suggest you put the events in chronological order. I'd be "hooked" enough by the "you never know who you'll meet in prison" thing.

Something like

Now, when you live in a prison, you’re going to find a lot of really strange people. Once there was a guy across the way who thought he was the Green Lantern. No joke. He even had a cheap, old ring on his right hand, the kind you get for a quarter in little plastic bubbles from the machine sandwiched between the two others that sell fake tattoos. The ring was harmless, or so the guards must have thought, so they let him keep it. They probably thought it was hilarious. It stopped being hilarious when they found him one morning dead, apparently having choked on his ring.

Up until yesterday, there was a guy in my cell who thought he was Jesus. A guard eventually told me that my...

Also, I'd shorten the description of the "Green Lantern's" demise: "The guards must have thought the ring was harmless, maybe even hilarious, until they found he'd choked on it one morning."

I mostly liked the MC's rambling except:

1) "sandwiched between the two others that sell fake tattoos"
This is a super concrete detail. But it is the straw that breaks the camel's back in the description of Green Lantern's ring.

2) "They probably thought it was hilarious. It stopped being hilarious when..."
Try combining these sentences into one to make the description of Green Lantern's death shorter.

3) "But I’m already off topic."
I will allow this guy to meander- but not if he knows that he's doing it AND tells me about it. In fact, this line sounded like the author's voice imposing itself on the MC's voice

With all respect to wbriggs, who had some very good ideas, I personally liked starting the narrative with the mention of the "Jesus prisoner" better than ordering the narrative chronologically. Starting with the "Jesus prisoner" cements the idea that he is the focus of the story- not Green Lantern.