first of all I'm so damn new with this so try to explain as simple as possible
in like every tutorial they say copy paste a background/render - how should I do this?
and, when I have a picture that's not cutted, how do I do? use one of the tools or? which

regards

pasting a render can be done several ways...the most common ways youll run into:
-you download a render pack so you can just open up a new layer and go to file and open the render wherever it is saved and drag it onto the new layer you made by pressing ctrl and dragging it
-the render image you want is online..if this is the case just copy and paste it into your layer

if the picture is not cutted but is rendered (meaning it will usually be on a solid color background such as black or white):
fast way is to just use the background eraser and set the threshold at whatever it takes to remove the background without removing the image or you can use the magic wand and then feather the image before you delete the background so its a bit smoother (for sigs i usually feather only about 2-3 pixels).

for the best possible render removals, i would use the polygonal lasso tool or the pen tool.

hope this helps.

This