The creator of a wiki (known as the founder) is automatically an admin by default.
Otherwise, to become an admin on a wiki you need to be promoted the permission through Special:UserRights on your wiki by a bureaucrat. If there are no bureaucrats around, and no admin has edited in the past 60 days on your wiki, you may attempt to adopt the wiki - you can read more about this here. Be sure to read through the guidelines before filing the adoption.
To make other users an admin, go to their contributions page and click "User rights management" - tick the "Administrator" box and save; the user is now an admin on that wiki. The same instructions may be followed except unticking the "Administrator" box to remove the administrator rights of said user.
Note that bureaucrat rights can only be removed by the bureaucrat themselves or staff/helpers - other bureaucrats cannot remove other bureaucrats.