The "founder" of a wiki, the person who first requested it, is given administrator access automatically. Founders are also given bureaucrat access so they can make any other user on their wiki an administrator or bureaucrat. For a complete list of users with administrator access, see Special:Listusers/sysop.
For you to become an administrator, someone with bureaucrat access must make you one. On larger Wikia, there will usually be a community page for requests. On a smaller wiki, you may simply ask on the community portal or Forum, or ask a bureaucrat directly on his or her talk page.
If there are no active bureaucrats, use the "Requests for adminship" page to nominate users to become admins or bureaucrats. Once a community decision has been reached (or if there is no active community to debate the decision), please leave a message at the wiki adoption requests page and Wikia Staff can provide the rights.
Administrators should not use their administrator powers to settle editing disputes; for example, to lock a page on a version he or she prefers in an editing dispute that isn't vandalism. Administrator powers should be used to help keep the wiki clear of vandalism, spam, and users who make malicious edits, but not for simple disagreements between users acting in good faith. Ideally an admin shouldn't be considered "in charge". The ideal admin is just someone who is trusted to have a few extra buttons and to use them for the benefit of the Wikia community.