Yes, they did know what would happen to them if they disobeyed God and turned their backs on Him, because He had clearly warned them. He said, "You must not eat from the tree of the knowledge of good and evil, for when you eat from it you will certainly die" (Genesis 2:17).
And yet they chose to believe Satan's lies instead, just as we do, all too often. Satan said to them that God had told them a lie, and they wouldn't die if they ate of that particular tree. (Incidentally, it wasn't necessarily an apple tree; the Bible doesn't say what it was.) Instead, Satan told them, if they ate it they would be transformed and become as powerful and as wise as God. What a temptation -- to become like God!
Why did they choose to believe Satan instead of God? The Bible doesn't exactly say, but they alone were responsible for what they did. And every day, you and I face the same decision: whether to believe what God has told us, or to believe Satan's lies. The Bible says, "I have set before you life and death, blessings and curses. Now choose life, so that you and your children may live" (Deuteronomy 30:19).
Thank God for His written Word, the Bible, and I commend you for wanting to make it part of your life. But as you read it, thank Him most of all for the Living Word, Jesus Christ, who gave His life for our salvation so we could be forgiven.