Trouble with SQL Server authentication in SQL Server 2012
New accounts with system administrator access can be created by other equivalent accounts but there is no way to reverse engineer the password for an existing account. You should also verify that the SQL Server you're connecting to allows for mixed authentication (Windows and SQL Server).

This StackOverflow question describes how to access authentication via SQL Server Management Studio (https://stackoverflow.com/questions/1393654/how-to-change-from-sql-server-windows-mode-to-mixed-modesql-server-2008)