Every megacorp owns one or more buildings (which are the same as cybersites at those locations), one of which is designated the headquarters for that megacorp.
Headquarters appear as cuboids painted with the megacorp logo when seen in cyberspace, which makes them easy to spot. In meatspace they look the same as other buildings owned by the corporation, but have a waving flag with the megacorp's logo at the top of the building.
Usually the headquarters of a corporation is the biggest building it owns.
You can find the headquarters of any corporation by clicking its info button, and pressing the "select headquarters" button at the bottom.
Some headquarters, such as that of 256 Shades of Hats, offer special abilities that you can't access from other buildings.
You have a limited sphere of influence, only within which can you sell adverts, buy buildings, and take other local actions. To succeed it's important to move and expand your sphere of influence.
You can toggle an overlay with your circle of influence with the "Influence" button at the top of the interface.
As the player, you can reassign your headquarters to any building you own. Your sphere of influence is always centered on your headquarters, so relocating your headquarters is an effective strategy for reaching new areas of the city.
The size of your area of influence is directly affected by the height of the building that is your current headquarters; as such it may be useful to move your headquarters to a taller but less populated building. Usually the tallest buildings in a given city will give you a sufficiently large sphere of influence to be able to affect the majority of the city.