There is no one singular answer to this question. People make decisions in a variety of different ways, people may not even know the process that they go through to make decisions. Some religious people may look to their own holy books or teachings for guidance. But what about non religious people? Where do they go for guidance?
This is a non religious group of people, who do not believe in any god, life after death or religion. They believe life is to be lived and there is no further purpose. This website here explains a little more about their beliefs and how they come to making decisions. http://understandinghumanism.org.uk/uhtheme/ethics/?age=14
Utilitarian is a way of thinking based on doing the best to make the most people happy. There are different types of Utilitarians and this video explains how Utilitarians make decisions.
If in an exam you are asked about how non- religious people make decisions. These are two examples you can use.