Hi 49, as Dino246 said, a more modern translation would help facilitate one's understanding of the passage.
NIV Genesis 4:1-7 1Adam made love to his wife Eve, and she became pregnant and gave birth to Cain. She said, "With the help of the LORD I have brought forth a man." 2Later she gave birth to his brother Abel. Now Abel kept flocks, and Cain worked the soil. 3In the course of time Cain brought some of the fruits of the soil as an offering to the LORD. 4And Abel also brought an offering-fat portions from some of the firstborn of his flock. The LORD looked with favor on Abel and his offering, 5but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast. 6Then the LORD said to Cain, "Why are you angry? Why is your face downcast? 7If you do what is right, will you not be accepted? But if you do not do what is right, sin is crouching at your door; it desires to have you, but you must rule over it."
I personally understand the passage to state that first, Cain's offering was not the first-fruits or quality, or of the right prescription from God i.e. it was an unsatisfactory or deficient offering to God, showing contempt. Whereas Abel chose the best that he had to offer. (as reneweddaybyday brought up Heb. 11:4, I couldn't agree more).
Secondly, God warned Cain that sin was crouching at his door, meaning due to his downcast face, God could tell what Cain had in mind i.e. take revenge on Abel.
Thirdly, due to God's perception of Cain's intent, he, in a just and fair manner, both warns him of the evil that is eagerly (desires to have you) knocking at his door, and gives him the means to resist i.e, assures him that it is within him to circumvent what he is about to do, that is, kill Abel.