Finding a dead body can frequently be a traumatic experience. Even when the body is not a loved one, personally encountering death can have wide range of effects on individuals.
The most important thing to do is to remain calm. Then, if you are able, confirm death without disturbing the scene or body, and contact the authorities (9-1-1). Feeling for a pulse, or breathing, will generally be okay. If the deceased has clearly been so for some time, there is no need to confirm a pulse or breathing. Additionally, you can call 9-1-1 before doing anything, and allow the operator to walk you through what to do.