Eve was the first one to eat of the tree of knowledge of good and evil. Genesis 3 informs us that Satan, personified as a serpent, lied to Eve saying, "You will not certainly die, God knows that when you eat from it your eyes will be opened and you will be like God, knowing good and evil." When "the woman saw that the fruit… was good for food…and also desirable for gaining wisdom, she took some and ate it." This account indicates Eve disobeyed God to gain wisdom. She believed Satan over God and was tricked. This is corroborated by 2Cor. 11:3 "…Eve was deceived by the serpent's cunning…"
Adam was not fooled into believing Satan's lie. He chose to follow Eve into disobedience. God says in Gen. 3:17, "you have listened to the voice of your wife and have eaten of the tree of which I commanded you, 'You shall not eat of it'…" Adam willingly took the fruit from Eve and ate it. He knew what he was doing.
Why did Adam decide to sin? The Scriptures don't further elaborate on this point; however, since Adam knew Eve was already in sin, he seemed to value and choose Eve's companionship over a relationship with God.