What exactly do you mean?
Are you trying to set it to where you are notified when someone replies on your thread? When someone replies on a thread you posted on?
Generally, your account will be set to where any thread you create or post on will automatically be added to your subscriptions. However, like PopsBWell said, you can click "thread tools" above the first post and subscribe to a thread without posting on it.
To view threads you're subscribed to click "settings" at the top of the page then click "subscriptions" on the left hand side of the settings page.
You can also set it to where you receive an email when a thread you are subscribed to has any new posts. To set this up, click "settings" at the top of the page, then scroll down and click "general settings" (it is on the left hand side of the page, under the *my account* section). Once you are on the general settings page, scroll down until you see "Default Thread Subscription Mode:" and change it to "instantly, using email."
Although I do think the whole email subscription thing is overkill.
I just use the "settings->subscriptions" to view any topics I've made/commented on/subscribed to.
what pops mentioned as well as yours both worked! thanks for the help, I was getting frustrated trying to figure out if someone had replied to anything on a thread i had posted! Thanks guys!