Mankind was given the right to choose right or wrong. But along with this ability, comes consequences, which are like rewards; choose to do the right things in life get positive rewards, while choosing to the wrong things will get poor rewards.
God gave us the ability to reason which in simple terms is called “free-will.” That explains how sin entered into this world. God told Adam, that he could eat the fruit of any tree in the Garden of Eden except the fruit from the tree of knowledge, and explained the consequences thereof. Eve, after being tricked by the serpent, chose to sample that fruit and found it to be good for consumption.
As a result of direct disobedience to Gods command, Adam and Eve were banished from the Garden forever.

Choices and consequences!