Calvin's theology is very complex and we can't begin to do justice to it here. Nevertheless, we can still identify a number of key ideas. Perhaps the most important of these is double predestination. This is the belief that God has determined in advance which people are going to heaven or hell. Traditional Calvinist theology holds that even before someone's been born, even before they've had a chance to do anything good or bad in their lives, God has already decided whether they're going to heaven or hell. One can be the most God-fearing Christian or the most incorrigible sinner; ultimately, it makes no difference as to your soul's ultimate destination. Only God can make that decision, and he made it before he created a single soul.
Another important idea in Calvin's theology is the utter depravity of humankind. This means that man, ever since Adam and Eve disobeyed God by eating of the Tree of Knowledge in the Garden of Eden, has been mired in sin. According to Calvin, people are not innately good; on the contrary, they are selfish creatures, always out for what they can get. Calvin doesn't deny that human beings can often show signs of goodness, but what he emphatically does deny is that such goodness is an intrinsic part of sinful human nature. Whatever goodness we possess comes purely and solely from God through the freely given gift of divine grace.