Creating a Mod?

How do you actually go about doing this?

For example if you wanted to create a mod that would add another box on the side which displayed selected items (or something along those lines, that would require additional filters for SQL statements etc).
What would you use to start off, duplicate certain files?

I notice some contributions have quite a few files in them, and I'm never entirely sure what they are all for.

I just want to know so I can modify my store in the best way possible, also that if i make useful mods i can add them as contributions to the community.

Cheers
Carlo