Genesis 4:7

"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." Genesis 4:7

That first question implies that Cain knew what he should have done with the offering and didn't do it.  Whether that was to offer something different or to offer a better portion of his crops then he did doesn't matter.  He knew and didn't do it.  Now God is warning him that one step into sin can lead to further sin unless he controls his sinful desires.

Just because we fail and commit a wrong doesn't mean we have to give up.  God wants us to succeed.  He'll send us warnings when we're starting down the wrong path.  We need to master our sinful desires, and when we can't master those alone, he will help us.