what you are asking is called blending or fading. to accomplish there is a button in the layer panel on the bottom between text style button and new layer set.
it is called "add layer" mask. use the brush tool to paint what is visible with white color. black color is not visible. any color in between varies the visibility of the image being masked.
you can also use the layer mask with the gradient tool and drag the gradient tool while on the mask. this is how mask is used:
=========================
instructions on using layer mask:
click on the thumnail on the layer panel. this layer is the one with the image you want to mask.
click the mask button
you now have 2 thumnail selections for the layer:
layer thumbnail lets you edit the contents on the layer. click this thumbnail to edit the content on the layer.
there is a chain icon meaning it is linked with the mask. that link can be disabled if you do not want the mask to move when you move the contents inside the layer with the move tool
the thumbnail on the right is the layer mask thumbnail. you must select this thumnail to edit the mask. click back on the layer thumnail to edit the contents on the layer.
use the airbrush, pen, ect to create the mask. any color can be used but colors closer to white are visible, colors closer to black make those specific parts of the image not visible. colors in between have various levels of transparency depending on shade of color
