Right click on server name in Object explorer tree at the left
Select properties -> Then choose the "Security" page at left
Under Server Authentication, select "SQL Server and Windows Authentication mode"
Click OK
Exit SSMS completely
Open "SQL Server Configuration Manager" (in Configuration menu under the "Microsoft SQL Server 2008 R2" menu item from start menu
Select you instance on SQLExpress and restart the instance of the server (but this is ok for non express too)
Exit SSCM - and to verify open SSMS again, but with "SQL Server Authentication" selected in the Authentication type dropdown - put in your SQL user ID and password.