First time plugin creation

I’d like to try to create a simple plugin for use on my blog. The comment would require me to add another comment form field – along with checking to see if it is filled or not before it’s ‘submitted’.

I’m not sure how to start really. I’m confused about how all this works.

Lets say my plugin would add another field to the comment form. How would I design a plugin to do just that? If I could figure that out, the rest would be pretty simple.

EDIT: I’m just confused about how all this ‘hooking’ business.