Find the confirmation message for the list you subscribed, click it and follow the instructions. If you don't see the confirmation message, then come back and check again in a few minutes (or a few hours).
Many confirmation messages have a link to a web page where you can confirm the subscription. However, if the confirmation message requires an email response, then you will need to send an email to the address in the message and follow the instructions for the subject line and message contents.
[Earlier messages] [Later messages]