One of the best parts of writing sci-fi for me is world building.
Most of the time, when I write something, it's set in our world with normal laws, both physical and social, although I usually deviate from some sense of normal. Sometimes that makes it easy, knowing where the limitations of what a person can or can't do. A normal person is not going to be able to jump two stories or go more than a couple of days without rest and food. These are just the natural rules of our world.
There are ways I've gotten around the rules of this world. One is by making the character somehow extraordinary to where they don't apply to him. But there are still laws that apply to him, such as social laws. Super powers don't give a person the right to say whatever they want. They still have to be able to function in this world in a believable manner if they are set in their world, planet, country, even region and time-period.
Another way to get around the world's rules is to make your own, which is what I'm having so much fun with. Creating a new world is the closest I get to playing God. You decide how people live, how they act and interact with each other and their environment. A good place to start is the geography, and from there (forgive the pun) you build the people from the ground up. Culture is very much affected by geography, and from there you get social norms and even the types of animals and hardships they deal with. How people live determines what their ideas of the purpose of life would be, and from there you can spawn a whole religion. You can design whole star systems and histories that shape these people's racial experiences.
I'm not going to lie, it might take a while. Cultures in real life take hundreds of years to develop and still constantly change with time, so its sort of unrealistic to come up with a fully developed culture overnight. My advice to new writers is to just get started, put something on paper, anything, but don't let it sit there, but build on it, and soon it will start writing itself. An idea is like a seed; it has all the potential in the world to be a big strong tree, but while it's small, it just needs some love.