Reading from a returned DataTable...

I wrote a class and method to return rows from a table in my SQL Server database. Not sure what I'm doing wrong. Could someone have a look at this code and let me know what I am doing wrong? Does anyone have any links to good articles on best practices on dealing with data access classes and database tables?
 // Here is the class

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;

/// <summary>
/// Summary description for DailyUsage
/// </summary>
public class DailyUsage
{
    private string ConnectionString;
    public DailyUsage()
    {
        ConnectionString = He ...