The relevant verses are Genesis 4:8-17.
I've created an elaborate, highly detailed table to summarize the consequences of Cain murdering Abel:
Cain before murdering Abel:
- Lives in Eden, which has already fallen.
- Can grow crops.
- Can be killed by humans.
Cain after murdering Abel
- Must leave Eden. Which, again, has already fallen.
- Can't grow crops. Can simply have others do it for him. One less job to do.
- Can't be killed by humans.
And that's really all there is to it. Verse 12 (NIV) also mentions that Cain was cursed as "restless wanderer", but that obviously didn't hold since he goes straight from Eden and lives in Nod (verse 16). Then at the end Cain settles down with a wife and kid (verse 17), builds a "city", and lives happily ever after - at least that's how it reads to me. Am I missing something? It seems like God essentially lets Cain get away with murder, and His "curse" doesn't do much of anything.