After setting up your Thunderbird client, you are ready to start sending emails.  To send an email, first click on the "Write" button at the top of the program:

This will open a new pop-up window that will allow you to compose your message.  Fill in the necessary information (the email address you are sending to, the subject, and the body of the email), and then click the "Send" button at the top of the window:

Congratulations, you have just sent an email with Thunderbird!  Once your email has been sent, it will now be in the "Sent" folder if you need to access it later or just need to confirm that the email was successfully sent.