Make Part of a Layer Transparent?

There are a number of different ways, but what I would suggest is clicking on the Add Layer Mask icon at the bottom of the Layers panel (it’s the third icon from the left, circled above). Now, you can take the Brush tool (B) and paint away any part of the layer (making it transparent). By default, the Brush tool is set to black, and that makes whatever you paint over transparent. But, unlike just using the Eraser tool (which actually erases things on a layer, and they don’t come back), a layer mask is non-destructive, meaning if you mess up and erase too much, you can just switch your Foreground color to white (press the letter X to swap your Foreground and Background colors; so, pressing X, at this point, would make your Foreground color white), and paint over those areas you didn’t mean to erase, and they come right back. That’s because you really didn’t erase them in the first place—you put a mask over them when you painted in black. If you didn’t want to fully erase part of the layer, but just make part of it a little transparent, then go up to the Options Bar and lower the brush’s Opacity setting a bit. The lower the percentage of opacity, the less it erases in one stroke. So, if you want to see through the layer a little, try a low opacity setting for your brush.