Satan, who came to Eve in the form of a serpent, mislead her by telling a "half truth". Would humans have become a god by eating from the fruit of the tree of knowledge of good and evil? No, but they would know about good and evil, like God: "(Genesis 3:22) And the LORD God said, Behold, the man is become as one of us, to know good and evil."
The devil also told them they wouldn't die by eating from this tree. This was not completely true, nor was it a plain lie. That's the way the devil works.... Physically, they would not immediately die, but spiritually they did die immediately. Their relation with their Creator, God, was broken by sin. And obviously this sin was: disobedience to God.
Why God placed such a tree in the garden? As God is holy and can not sin, He absolutely didn't want us to be sinners and make Adam & Eve sin. Why would He do that for? So, in my opinion (and if anybody has another opinion about this, please reply to this post) God placed this tree there not really to "test" if Adam and Eve would disobey Him but rather because He wanted to give human beings the freedom to choose to know good and evil and as a result of that die or obey God and live eternally. The two well-known humans in the garden chose to know good and evil. Therefore, they lost eternal life and the relation with God was broken.
Genesis 3:21 says: "Unto Adam also and to his wife did the LORD God make coats of skins, and clothed them." God had to kill an animal in order to clothe the first two humans on earth. Blood had to be shed because of the sin of the humans: disobeying God. God can't have a relation with a person who is bad. God, who is holy, made Adam the first human being and therefore Adam was good. His nature was good. But by sinning this changed. All humans, descendants from Adam, are born with a bad and sinful nature because of the first sin that occured in the garden of Eden. This is what's called original sin: all humans are cursed by God and are born in sin and are lost (can not belong to God's Kingdom and go to heaven).
The Israelites had to bring sacrifices to God in order to be forgiven their sins (disobey God's word). God gave them the law which they had to obey completely. This, however, was nearly impossible to do. God's people, Israel, couldn't directly pray to God to forgive their sins without having to shed the blood of an animal. Do you see the relation to what happened in the garden of Eden? The core of the Gospels and the reason Jesus came down to the earth has everything to do with the story of Adam and Eve. If you want to know about that, I can tell you (although it's rather difficult to talk about that in English as it's not my native tongue).