Normally, when you reply to a thread, you are automatically subscribed. That is the way I have everyone's setting defaulted to when you first register. You can change your options by going to "My Profile" and "Edit Options" and then "Default Thread Subscription Mode."
You can choose:
- Do not subscribe
- No email notification
- Instant email notification
- Daily email notification
- Weekly email notification
If you like lots of emails, at the top of a forum section, under the Forum Tools option, you can even subscribe to a entire forum.
You can elect to subscribe to particular threads of interest, even if you haven't replied to it, by selecting the "Thread Tools" pull-down menu.