To create a layer mask in Photoshop:

1. Show your layers (WINDOW / LAYERS)
2. Select the layer you’d like to mask
3. Click on the LAYER MASK icon (  ) at the bottom of the LAYERS panel
4. Select the BRUSH TOOL on the toolbar and select your BRUSH from the preset brush picker at the top
5. Put BLACK as your FOREGROUND color at the bottom of your toolbar
6. Ensure that you are in the THUMBNAIL (the mask on your layer) and paint with your BRUSH in BLACK
7. To undo any areas, select WHITE as your FOREGROUND color and paint with your BRUSH in WHITE
8. To create translucency, select a shade of GRAY and paint with your BRUSH in GRAY

Remember in the layer mask:
BLACK = TRANSPARENT
WHITE = OPAQUE
GRAY = SEMI-TRANSPARENT

For best results, I encourage you to watch the short quick tip video above as many times as you need!