How to activate email notification?

I'm probably too sleepy but I can't find how to activate the email notification when a reply is posted to a thread can someone help me?

Look for "subscribe to topic", i think that was it. And if you do reply to a topic, it should also subscribe to it.

When you reply there is a check box at the bottom

Check this box to be notified of replies to this topic.

You have to check it each time I think.

Thank you,
the subscription was set but I don't know why the email was not sended, now it works correctly, for the replies at this thread was sended correctly.

One more thing, the system will send one set of e-mails, after that it will not send anything until you login again.