I want to write a plug in.

I have read some of the documentation about how 'hooks" work but I do not know which hooks to take advantage of.

I want to write a plugin that launches a separate page when a new user wants to add a new comment.

In other words, I want the ability to insist that the user add more than just a username and email.  I want to require that more information be inputted.

Maybe such a plug-in already exists or perhaps something simular.  Does this sound familiar?