Hello, I am trying to get into making "party" flyers for my friends, and I was wondering one of the most basic things.

How do I work with layers, so that a picture looks like its behind another, etc?

On the right side there is a panel with layers.
When you start a new document, it will have on saying "background".
You can make new layers by pressing ctrl + shift + N, and then click and drag these layers in the layer panel (bottom is background, top is foreground).

Oh so how do I add layers like new pictures? just drag them in there?

Copy and paste.
By default, pasting something in to a layer that already has something in it will create a new layer with the pasted object.
You can merge layers by highlighting multiple (shift + click on layers) -> right click -> merge.

Ctrl + alt + shift + E will make an applied image (a flattened version of every other layer) on a new layer.

Click on the eye symbol in the layer palette to show/hide layers.

Thank you! Is there any sites you can reccomend that has good pictures or anything that may be able to go on flyers?

Nope, I've only got experience making tags.