This always seems to come up...
Example:
Say you followed someones how-to on how to install program "Foo", and now you have a problem using program "Foo"... Should you...:
A - Post a new topic asking your question?
B - Reply to the end of the topic where the how-to was posted?
If you answered B, you are right
By replying to the end of the thread, all the people that are watching that topic will get an email notifying them of a reply. Your chances of getting an answer to your question are much better that way.
Thanks,