She did not say that she had gotten the ^first^ man. There may have been others.
The differences between Cain and Abel and what happened between them is instructive so they are talked about in scripture.
What is the instruction? There are 2 fundamental types of humans -> the first type is carnal -> the second type is spiritual.
Since the Lord created man for intimate personal relationship and the fact that the Lord is Spirit, the CarnalMinded completely miss their purpose in existence.
The SpiritualMinded however seek after the Lord and desire to be in fellowship with Him.
Cain would offer a non-spirutual offering to the Lord as appeasement.
Abel would offer a blood sacrifice in acknowledgment of the sin that separated him from the Lord in an attempt to be reconciled to the Lord.
Those who seek after the Lord will be blessed and I suppose Abel was prospering more than Cain.
Because Cain was carnal he was jealous of Abel^s prosperity. This jealousy turned into covetousness and hatred resulting in murder.
In Cain we see a person breaking the first, sixth and tenth commandments which had not yet been expressed in words.
We see this sad event causing a permanent division on the first family. We see a curse hanging over Cain because of sins.