How to configure a database connection (using SQL Server in this example):

    //The connection string to be used in web.config file.
    //It is configured to use SQL server express 2005 version.
    <connectionStrings>
        <add name="myConnectionString"
             connectionString="Data Source=LOCALHOST\SQLEXPRESS;Initial Catalog=databaseName;Persist Security Info=True;User ID=sa;Password=123"
             providerName="System.Data.SqlClient" />
    </connectionStrings> ... 

How to use: 

     SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["myConnectionString"].ConnectionString);

    conn.Open();
    ....//any code...
    conn.Close();