What are some of the important criterias to consider before granting a credit to a customer? After then,there is a zero risk?

The ability of a customer to pay back is number one risk you consider as a financial institution. If a customer have a history of default he is considered a high risk. 

There is nothing known as a zero risk, but there is minimum and high severity and frequency. Because risk is usually measures as a possibility or a percentage and not an actual figure.