Checking Thunderbird is extremely easy.  After downloading the Thunderbird client, you should have a desktop icon that says "Mozilla Thunderbird".  Just double click this icon, and enter in your password.  Your email should automatically update as soon as the password is entered.  To refresh your list, click the "Get Mail" icon at the top left hand corner of the page.  To exit the email client, just click on the "X" at the top right hand corner, or go to File, and Exit.