We read the account in Genesis 4:3-7.
“In the course of time Cain brought some of the fruits of the soil as an offering to the LORD. But Abel brought fat portions from some of the firstborn of his flock. The Lord looked with favor on Abel and his offering, but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast. Then the Lord said to Cain, ‘Why are you angry? Why is your face downcast? If 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 master it.’” (Emphasis Added.)
Notice that the text says that “Cain brought some” of the fruits of the soil and God told him, “if you do what is right, will you not be accepted?” In other words, because Cain did not bring the best of his first fruits, his offering was not acceptable to the Lord.
On the other hand, “Abel brought fat portions from some of the firstborn” and “the Lord looked with favor on Abel and his offering.”
Abel’s example to us is that of returning the best of the first fruits that we have been given by the Lord. In so doing, the Lord will accept our offering and will give to us even more.