Open the Layer Style and choose Blending options by doing one of the following:
Double-click its layer thumbnail in the Layers panel.
Choose Layer > Layer Style > Blending Options.
Please keep in mind that the Blending sliders won’t work on a locked Background layer; you have to double-click the layer first to make it editable.
Drag the right side of the first slider, until you achieve the desired transparency. Works best with white backgrounds.