1. With an image open in Photoshop, click the top layer in your layer stack (in the Layers palette) to make it active.
2. Either -
From the pull-down Menus, select
Layer >
New >
Layer
Or -
Click the
Create a new layer icon at the bottom of the Layer palette. It's the icon that looks like a little sheet of paper with a folded-over corner.
You now have a new transparent layer at the top of the stack.
I can't check at the moment (don't have
PS on this machine I'm currently using), but I'm pretty sure this kind of thing is covered in the Help file.