Vs. 1 and 2
Here we see the birth of Cain and Able, while it is the first birth recorded it is not necessarily the first birth, remember that when God punished Eve He told her that “I will greatly multiply thy sorrow and thy conception” which to me indicates that there were children born while they lived in the garden, (how to you multiply something that hadn’t happened yet) we have no idea how long they were in the garden before they fell. We do know that Adam was 130 years old when Seth was born at the end of this chapter.
Never the less it seems clear that Cain is the first son born after the fall and Able the second.
Vs. 3-5
Cain and Abel bring the fruit of their labors and make an offering to God, He respects Abel’s offering of sheep but rejects Cain’s offering of grain. If you are like me you are wondering how they knew that. While we cannot know for sure here are two possibilities;
 (1.) God is seen talking to Cain in verse 6 so it would not stretch credibility to think that God himself told them what was acceptable and what was not, perhaps even before they brought them and maybe Cain was disobedient. When we get there we will see that when God is making the rules for Israel, there is a specific sacrifice that calls for grain so perhaps it was not the grain but the disobedience?
 (2.) There is a line of thought that says that when they made their offerings God sent lightening to consume Abel’s and not Cain’s. This seems logical as well since this actually happens 3 times that I know of, in Lev 9:24 in the wilderness , 2Ch 7:1 when Solomon first built the Temple, and 1Ki 18:38 when Elijah was on Carmel challenging the false prophets. It seems there was another time when a king of Judah cleansed the temple but since I can’t find it right now we will have to see when we get there ;-). Never the less it wouldn’t surprise me at all if this were true in the case of Abel’s sacrifice at this time as well.  
However they knew Cain’s countenance fell , today we might say he got the blues or had a long face but however you say it he was not pleased but notice in the next verse that God comes to Cain to help and warn him.
Vs. 6 and 7
God asks “Why art thou wroth?” “If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him. “
First of all why are you mad? Which seems to support the thought that Cain had some prior knowledge of what was acceptable and what was not and just didn’t do what God had asked.  Then God asks  if you do well won’t you be accepted but then warns him if you don’t do what is right sin is waiting at the door, and unto thee shall be his desire and thou shalt rule over him.
A few thoughts;
(1.) It seems that what Cain did wasn’t what God desired but wasn’t sin either, Cain did bring a sacrifice which seems to indicate that he wanted to please God.
(2.)  When God didn’t acknowledge Cain’s attempt to please Him Cain became upset and very vulnerable to satan and his lies.  I don’t know about you but personally I have been in this position before myself with varying results.
(3.) God told him “sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.” Which seems to me to say that when sin comes knocking we are not supposed to answer. Notice also that God called sin his and him not it. Perhaps a clue that all sin is inspired by satan and his fallen angels.
Vs. 8
Cain succumbs to temptation and kills Abel. It occurred to me during the last chapter that this might have been inspired by satan because God had just told him that he would be judged by Eve’s seed. How better to prevent that than to cause one son to kill the other thereby disqualifying them both. See satan knows the rules which I believe are universal and everlasting, and he knows how to manipulate others into breaking them, like the mob boss that never kills anyone but had scores killed, so they get punished instead of him.
Vs. 9-16
God responds to Cain’s action in a similar way as he did to Adam and Eve in the last chapter but Cain doesn’t, he won’t admit it until God lets him know that He knows what Cain had done and unlike Adam and Eve who were punished but not cursed Cain received both punishment and curse. Cain complains that he is being punished to harshly and God puts a mark on him so that nobody would kill him. Seems like there is a figure in our days that has a mark on him as well, someone named Harry Potter, hmmmm.  Anyway he is driven out of the land of Eden even further east to the land of Nod.
Vs. 17-24
Here we see some of the consequence of Cain’s sin.  Cain finds a wife and has sons. The first one named Enoch (not the one we will study later) built a city, the beginning of government, God had said to spread out and fill the Earth not band together and build cities. This Enoch’s great great grandson Lamech (again different from the one we will study later) had 2 wives, the beginning of bigamy and later kills a man and boasts about it by mocking God’s protection of Cain from vengeance. Out of Cain’s line came the inventers of music, nomadic herders, and metal forging.
Vs. 25 and 26
Here Adam and Eve get another son and notice that Eve says “For God, said she, hath appointed me another seed instead of Abel” perhaps She believed Abel was that seed that would crush the serpent’s head as well. Seth has a son named Enos and “then began men to call upon the name of the LORD.” Same as before YHVH or Jehovah.  Though I am not sure what all this means it seems that before this time they didn’t call on God by name but rather by Eloheem as in chapter 1.                                                                                                                      Here at the end of this chapter I would like to point out something that I will explore more fully later. In the first chapter the word God or [Eloheem] is used exclusively to describe God. In the second and third LORD God [YHVH Eloheem]  is used and in the fourth LORD [YHVH] is used by itself except when Eve is quoted in verse 25 where God is used by itself as in the first chapter. My point is this, I believe that this part of Genesis was written by these people and saved through the flood by Noah, there are several clues that I will get into later but this one of the different names of God has just occurred to me today and fits with the others well so I am pointing it out now before I forget. One thing I have learned somewhat is that unless I write something down when it becomes evident I will forget it all to often. Once I heard a prominent evangelist/ teacher say that there was something that was smarter than anyone, a piece of paper because it never forgets what you write on it.