You need a certificate if you want to fly an unmanned aircraft outside of the Part 101 rules - such as operating unshielded at night, or with an aircraft that weights more than 25 kg.
Part 102 Unmanned Aircraft Operator Certification outlines how to obtain an unmanned aircraft operator certificate.
Part 102 is assessed on the risk of the operations you will be conducting. To operate with a variation from the Part 101 rules, you will need to show how you can manage the risks associated with that particular activity.
You will be required to provide us with a series of operating manuals showing how you have identified the hazards and risks of your operation, and the ways you will mitigate those risks. Each application is considered on its merit.
Having determined that you need to be certificated for your type of operation, you need to fill out the application form and the Part 101 and Part 202 compliance matrixes. Find these on Advisory circulars.