To find a building site, open the Starmap and search for an empty Hexagon shape.
Fly towards that point to build an outpost.
Select the type of building you want to construct.
Make sure that you have met all the requirements needed and press X (PS4) | A (Xbox One) | B (Switch) to build it.
Note that some outposts may have additional prerequisites such as Alliance progression. For more information, check out this article HERE.