This will save the file to your Arts & Media folder!

That will cause Photoshop to make a selection around the tree.

It's a pretty rough selection

At the bottom of the layers panel, click on the layer mask button

Now we have a layer mask

This is ok because we are going to be able to refine that in our select and mask workspace

This is a separate interface from regular Photoshop

MAKE SURE YOU HAVE THE LAYER MASK SELECTED NOT THE IMAGE THUMBNAIL

(You should see a WHITE box around it!)

Choose SELECT AND MASK

In Select and Mask mode, there are alternate tools and properties

Click the View pull down in the properties panel

Select On Layers

Drag the RADIUS slider to the right (to about 140) 

What happened?

Notice that now we can start to see through the tree to the lake

Once we are happy with the mask, we need to output a refined layer mask

On the right, go to output setting and click on output to

Choose New Layer with Layer mask 

If you click on the refined mask, you can see a lot more detail!