To crop one of your layers, you'll want to first select the layer. Once you've done this, you should see a property bar appear above the layer on your canvas. Clicking the leftmost icon on this bar will allow you to crop your layer to your desired dimensions.
Once you've adjusted your dimensions you'll see the option to to save your crop, undo your current crop, or reset your layer's dimensions.
Note: Not all layer options have the ability to crop.