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. (Genesis 4:7)

What can we learn about Sin from this verse alone?

Firstly, his desire’ reveals that sin is a person.
Since God is not accepting Cain's sacrifice, we understand that this sin is not of God, it is an evil, unclean spirit.  

Secondly, "Lieth at the door" reveals, this evil spirit is in waiting, for any believer whose offerings are not accepted by God. Any believer who disobeys God's Word is always in danger of sinning. Jesus is the door, any one who gets out of the Door i.e. Jesus, will need to face sin.

I am the door: by me if any man enter in, he shall be saved, and shall go in and out, and find pasture. (John 10:9)

Thirdly, "his desire" reveals, sin being an evil spirit is looking for a body to reside in, and therefore desires to dwell in him.

Fourthly, not just to dwell in him, but to "rule" him and takeover his nature. God tells Cain that he needs to overcome sin. Fallible man cannot overcome sin, only way to win over sin, an evil spirit is through Jesus alone.

In Mark 5:1-20, we see Jesus encounter with a man who had over 2000 unclean spirits dwelling in him. This man could not be restrained, he could easily break the chains people bound him with. The evil spirits ruled the man, after overcoming him.
Jesus delivered the man completely and everyone marvelled at the great transformation. 

Neither yield ye your members as instruments of unrighteousness unto sin: but yield yourselves unto God, as those that are alive from the dead, and your members as instruments of righteousness unto God. (Romans 6:13)

Sin waits for us to fail. It waits for Christians to be careless about their walk with God. Abel's offering was respected by God because he cheerfully gave his best. Whereas Cain, though he too offered, it did not please God. He did not give the best of what he had.

Believers, followers of Christ, should not wilfully go against the word of God. Neither live religiously without a heart for God. Instead, live each day doing the will of God, joyfully, overcoming sin.