How do I do it?

go to Windows > Layers

 

to open the layers window. then use the "opacity" slider and or the "fill" slider to adjust the transparency on an image/layer.

 

http://www.dodart.net/uploads/exapmle.jpg

:\ I've got a picture, but it's only one layer :o Do I have to make layers, or can I just select the area which I want to make transparent. It's only a couple corners...

if its just one image one layer you should still be able to make it transparent with the opacity and fill sliders. however if the image is "flattend" then what you want to do is just select the image>hit Ctrl A then Copy>file new>paste. then the image shouldn't be flaten'd anymore.or you can just right click the image>duplicate layer. and the top layer image will have a transparent backdrop wich will allow you to make the image transparent....i'm new to wrighting photoshop tutes, so if this doesn't help you and or you cant figure it out then send the image to me and i'll do it for you.

I'd explain the color table stuff but I'm too lazy right now.

255 colors with the 256 being transparent.