As far as I know, you need at least, to be Local Admin or part of Windows Admin Group on the machine running SQL.

You need to stop the instance, and start MS-SQL on single user mode. Once you start on single user mode, connect using sqlcmd and add yourself or whoever needs it, to the sysadmin server role, this way:

EXEC sp_addsrvrolemember 'CONTOSO\Joe', 'sysadmin';

GO

Now, all those actions are really intrusive: stop sql, star sql, override permissions, etc. So be sure you have permissions for that. Any attempt to do this without proper authorization or informing your bosses or right department, may cause you serious problems, in some cases, loss your job.