Man was created in the image of God, this reflects the special nature of man in that at creation man shared certain attributes of God. Man had some degree of dominion granted to him and could likewise recognise the dominion of God. Man also had a free will and was able to both sin and not to sin, he had a moral ability to chose his course of action.
The creation of man was decreed by God in order to show his glory, and this glory was decreed to be shown by his plan of salvation. In order for God's glory to be exhibited through his loving and merciful plan of salvation man had to fall in order to be in need of salvation. Therefore according to the will of God man sinned and ate of the tree of knowledge of good and evil. From that moment the image of God in man was marred and man became his own moral authority, unwilling to accept the laws of God.
Man became unable to do anything on his own that would please God, man retained a free will but now that will is only able to sin, whatever option he may take the result is sin. As Martin Luther explained mans will was free but it was now in bondage to sin. Man's actions, indeed all actions are decreed by God however this does not mean that man does not have a free will, man always acts according to his will to which God does no violence. He is fully accountable for his own actions as he acts according to his will with his own full consent. As George Whitefield memorably said to John Wesley:
“Man is nothing: he hath a free will to go to hell, but none to go to heaven, till God worketh in him to will and to do his good pleasure”
One of Calvin's most distinctive beliefs was that man was an idol factory, man seeks to create idols at all opportunities and therefore all attempts at religion from man just result in him creating idols and all attempts to comprehend God result in creations from our own minds.
The doctrine of original sin is that all mankind inherited sin from adam, who is mankind's federal head, therefore the sin of Adam is inherited by the whole human race. This seems unfair to our fallen sensibilities as modern culture tells us that each individuals actions are how he should be judged rather than the actions of a remote forbear, but just as we inherit a marred image of God and an inability to please God we also inherit sin. This is also why Jesus is referred to as the new adam, as he is the new federal head of the church, as we fell an adam we are redeemed in Christ.
Man is at enmity with God and sinful man cannot withstand the holiness of God, the natures are opposed and cannot co-exist, God is a just God and mans rebellion must be punished. In this punishment God will be glorified as his holiness will shine the more brightly when seen in contrast with mans sin, the justice of God will be evident and the mercy and love of God will be shown in the salvation of his church.