Editing the Comment form inputs, and adding new ones

Hi all. Just as a disclaimer: my knowledge of WP innermost frameworks and PHP is below beginner level here.

I am simply wondering how to edit the default Comment forms on WP: I just want to change a field. I’d also love to hear recommendations for Plugins that achieve this effect. Surprisingly, all Google searches on this topic yield how to change the code, but I can’t find any plugins.

Anyway, my issue is that if I do have to go in and edit the default Comment forms. I cannot find where to do it. I am using Thematic. I was reading a tutorial from WP Tuts on this very issue, which is here for reference, but I cannot find where to insert/edit any of the code they are mentioning.

Any help would be appreciated. I am sorry this subject matter is so novice.