There are a few different ways that you can make a selection transparent in Photoshop. One way is to use the eraser tool. With the eraser selected, simply click and drag over the area that you want to make transparent. Another way is to use the opacity option in the layers panel.
Simply select the layer that you want to make transparent and lower the opacity until it is at the desired transparency level. You can also use the fill option in the layers panel to fill a layer with transparency. Simply select the layer that you want to make transparent and click on the fill option. In the fill dialog box, select “transparency” from the drop down menu.