Open Thunderbird (by double-clicking on its desktop icon or from the programs list accessible from your Start menu).
If it's the first time you've used it, it might possibly invite you to start setting up your email account details. If not, you should see a 'Choose what to set up' panel where, obviously, you choose 'email' to register your account details with the program.
You might find that Thunderbird will do all the hard work for you (because it recognises an ntlworld email address and already knows the appropriate server settings), If not, you'll need to use the option to fill in the details manually. When you do so, if you're given the choice to choose between 'POP' or 'IMAP' access, choose 'IMAP.
The incoming (IMAP) server needs to be set to imap.virginmedia.com, with your username entered as your full email address. SSL needs to be enabled, using server port 993.
The outgoing (SMTP) server needs to be set to smtp.virginmedia.com, with your full email address as your username and your email password entered into the appropriate field. The authentication type is Password. SSL needs to be used, with the server port set as 465.
Hopefully though, you won't have to enter all that lot yourself, as Thunderbird might be able to do everything automatically, with you just entering your email address and password.
Good luck!