Genesis 4:7 (New King James Version)
If you do well, will you not be accepted? And if you do not do well, sin lies at the door. And its desire is for you, but you should rule over it.”

I think by doing well he means doing what is right in the eyes of the Lord and believing in him.
God loves us when we love and believe in him that is what God wants.

John 16:27 (New King James Version)
for the Father Himself loves you, because you have loved Me, and have believed that I came forth from God.

this is a verse I have been memorizing and it backs up what I just said.

But this verse says if you do not do well, sin lies at the door.
I think this is talking about temptation.
When you are tempted sin is waiting, knocking on your door.

When temptation enters our heart we either master it and rule over it, or it causes sin full actions.

Like Cain, Cain knew God was displeased with him and pleased with Abel. Jealousy and anger crept into his heart. God warned him sin was waiting at his door. This was not a time to take temptation lightly. Now was the time to master his temptation before it got the best of him.
But he did not, he opened the door to sin and killed his BROTHER Abel.
How sad just because he did not master his temptation.