Do you mean actual email, or a Private Message? If it's an email, it should say the address of the user who sent the email to you. Copy and paste that into the "To:" line and then type your reply and send it just like if you were sending an email to anyone else. A private message, you just hit reply as if you were responding to a thread.
Personally, I just rant and rave, jump up and down a bunch, cuss a bunch, then have another beer, and sit back down and read more forum entries