Pretty basic question.. I kno.., I just don't know how its done.

I see it everywhere. I want to be able to sort out my content in my websites by most viewed, most commented, most popular, best rating, etc...with a drop down list, check box, etc... but I don't know how "sort by" works? Which scripting language should it be written in? Javascript? PHP? Can it be done with XML? Do I have to have a database set up like MySQL? Is there a good tutorial anywhere that explains it? I need to be explained the exact process of how it works. As you can see I really need direction.. Your help is much appreciated :D