Adam and Eve conceive two children, Cain and Abel. Cain becomes a farmer and Abel becomes a shepherd. At some point in time they both bring offerings to the Lord - Cain offers some of his crops while Abel offers some of the firstborn of his flock. God is pleased with Abel's offering but less than enthused about Cain's. Out of anger Cain kills his brother in the fields.
Q & A
Why was God not pleased with Cain's offering?
While Genesis 4 doesn't explain exactly why God was not pleased with Cain's offering (initially it seems to have something to do with veggies verus meat), Cain's subsequent behavior and God's handling of Cain's actions hint that God was not pleased with Cain in general; the type of offering didn't have much to do with God's displeasure. For instance, Cain has just murdered his own brother, and when God curses and banishes him all he can do is worry that whoever finds him will kill him.
A few other books of the Bible provide reasons (mostly in the form of character profiles). The author of Hebrews indicates that Abel offered his flock out of true faith while Cain offered his crops as a mere formality, and not out of faith. In 1 John, John states that Cain "belonged to the evil one" and notes that Cain's actions were evil while Abel's were righteous.
When God banishes Cain, Cain worries that "whoever finds me will kill me". God puts a mark on Cain so that he would not be killed by strangers. What is this mark? Several other books note the use of marks of protection. In Ezekiel 9 God commands a man to run through Jerusalem and mark the forheads of people who grieve the sins taking place there; Angels are to eliminate everyone in the city except those with the mark. In Revelation 7 angels are to harm the land and sea, but not until the servants of God have had "seals" put on their forheads.
Where is Everybody?
Cain is worried that whoever finds him will kill him. Yet Cain was the first human to be born...presumably there aren't too many other people around yet. If there are, where did they come from? They must have been Eve's, for Cain hadn't had any children yet, and Abel was dead.