To change the authentication mode follow these steps:
1. Connect to SQL via SQL Server Management Studio.
2. Right click on the server name on object explorer
3. Select Properties
4. Select Security
5. Change Server authentication to "SQL server and Windows authentication mode"
6. Click OK
7. Restart the SQL Server instance
To restart the server instance:
Click yes on the user access control dialog (if one pops up, not pictured) and the Microsoft SQL Server Management Studio confirmation dialogs.
See also:
How to resolve: Login failed for user 'qie'. (Microsoft SQL Server, Error: 18456)
How do you set up a database connection using windows authentication?