If you want to use a username that is already in use usurpation is the process by which you can get it.
For a rename request to an already existing account the following conditions need to be met:
the requester has been registered for at least 3 months at the time of request and has a cumulated edit count across all Wikimedia wikis of 400 or more;
the registered name of the requester is connected to the single user login system and he is the holder of the global account;
the requested new username is not connected to any non-deleted edits, except for edits on the user page and other edits that violate site policies (vandalism, copyright violation, etc.);
the holder of the requested new username is notified on his talk page and via e-mail (if she has enabled the option in their preferences), after which the user will be given a week to respond and choose a new name for herself;
where appropriate the global renamers and stewards may deny a usurpation request that meets the above criteria;
renaming may not be possible due to technical and other constraints;
currently it is not possible to usurp a username from a user that has substantial edits, or to unify accounts where the global account is held by a different person, except where the holder of the chosen username consents to transfer the name.
The requests to change your username should be made at WP:RENAME.