Layer Mask

I took this photo of off Google and removed the background using the quick selection tool and magic wand tool. When the background is selected, pressed the layer mask tool at the bottom of the screen. When the layer box appears, press the invert button so the background is removed then adjust the feather bar to how you like it.

I found a background on Google and pasted it into the document. I then dragged the image above onto the background. 

That then made the photo below.

Background Eraser Tool

I used the background eraser tool to remove the background from the trees. The left shows me in the process of removing and the right side of the picture shows you how the sky was before.