Not stupid at all! When I was first starting out in Photoshop, this baffled me quite a bit.

I'm glad you asked this, because I've been meaning to put together a thread of PS/graphic-making basics for a while and this could be a good start.

Anyway, back to your question. I'll start with where to find them:

1. You can find textures ANYWHERE. If you just look up "photoshop textures" in google, you will find SO many results. Many people will upload textures they've made to DeviantART and other online art communities. You can get them from livejournal, private blogs, etc. Since tumblr has sort of recently exploded in popularity in the past few years, there's an enormous amount of resources people have posted there. There are also a ton of tutorials and FAQs and such.

2. How do you use textures? There are a ton of different ways. You can absolutely use them as a background, but you can also use them to create different effects on your graphic. Generally, I categorize my textures into ones that are really colorful (these tend to be gradients or tie-dyes or any other sort of colorful image), grungy (generally these are black and white, and are used to make my graphics look dirty or scratched), light textures (generally bright colors that look like light flares on a black background - used to create lighting effects on graphics), and ones that I would use as backgrounds.....like....if I would use it as a desktop background, then I would use it as a graphic background. That probably didn't make sense.

Anyway, if I want to change the coloring of a graphic, then I'll put one of the colorful ones or a light texture on top and change the setting to soft light or overlay or screen or something. It all depends on how much of the color you want to show through and such. If I want to make my graphic look grainy or scratchy or dirty or something, then I'll use one of the grungy graphics and set it to either screen (if it's mostly black with white/light grey stuff I want to show up) or multiply (if it's mostly white/light grey with black stuff I want to show up).

This is definitely a very poor explanation, and I'm sure Jenn or another graphic maker would be able to explain it better (it's also 1 AM here....haha), but please feel free to ask more questions if this doesn't make sense! I explained it in a manner that sort of assumes you have a little bit of knowledge regarding photoshop (which I'm not sure whether you do, so I apologize if you don't).......so apologies if this just sounds like gibberish to you. haha.