How did you installed your SQL Server database? Check whether you have used Mixed Mode Authentication while installation. In mixed mode authentication use 'sa' as User Name and give your desired password.
Connection string for this would be
<configuration>
<connectionStrings>
<add name="conn" connectionString="Server=myServerName\SQLEXPRESS;Database=myDataBase;User Id=myUsername; Password=myPassword;" providerName="System.Data.SqlClient"/>
</connectionStrings>
</configuration>
Also check for any firewall setting present on server machine which may prevent connection to your database.