When you have made your selection and you have the proper layer selected in the layers palette (here, the layer with the wall if there are other layers)

then go to Edit: Copy.

This will retain that data of the selected area. Now go to File: New,

Press OK when the dialog box comes up to create a new document.

This feature (of copy and paste) has probably been around since the beginning of digital image editing and Photoshop 1.0 I'm sure. It's good to learn the shortcuts Ctrl C, Ctrl N and Ctrl V.

It's important to note that when you copy a selected area the new document will be created based on the dimension or proportion of the selected area on an X and Y basis.  Because the selected area reached the vertical and horizontal limits of the original image, the new document will be created with the same dimensions as the original document. If you want to create a new document you can either Image: Duplicate or select a layer that covers the entire image dimensions such as the background layer by Select: all and making sure the background layer is selected in the layers palette and then you can create an exact dimension copy.

Now Edit: Paste 

and you can see that we did select the ‘background'. When you paste it will automatically put that selection onto its own layer.

Well what if we made this selection of the background but wanted to select the woman as the subject? (Usually the idea but this is training to get you fully versed...)  Easy, just right click (when on a marquee or lasso tool) in the original document and then you can choose Select Inverse.

This will now select the areas that haven't been selected which in this case happens to be the subject and the dead palm trees.

When making selections you'll most often want to select the easiest way and use the tools together to get a selection. It all depends on the circumstances. With difficult backgrounds sometimes there is no easy way. The advanced art of the selection will be covered in depth in the upcoming AdvancedPhotoshop.com in the next few months.  There is plenty of selection training in the Basic Photoshop training program.

Go to the moVe tool in the upper right corner of the toolbar and drag this selection (the main subject layer) into the new document. Here in the layers palette you can see that I have both of the separate selections on their own layers.

After you've made a selection and put it on it's own layer and you plan on making more changes to it; you might want to first make a duplicate copy by dragging it to the new layer icon and work on this version of the layer.

With the eraser tool you can go in and clean up areas of the selection that maybe got skipped over before.

Remember that you can change the size of the eraser brush in the options bar or by using the [bracket] keys.

On the duplicated layer go ahead and erase the dead palm trees (or whatever is on your sample image). Photos.com doesn't want me actually distributing these images but you can get these same ones with full access from their website. It's well worth it if you're at all serious about Photoshop and especially graphic design.

You can use some more of the basic selection techniques such as right clicking and choosing Similar or grow after using the magic wand tool to select areas of blue that might be remaining.

In using these, once again if you have accidentally selected areas that you don't want selected, you can use ‘subtract from selection' in the options bar of the lasso (or marquee tools).

Then simply lasso around the area you want de-selected.

See how easy it is for you to learn? Do you have any idea how long it took me to fully understand this simple concept?  Please use the leverage of other people's knowledge to get you where you want to be. You can leverage my guruistic knowledge of Photoshop with the Basic Photoshop DDV training program.

You can use the eraser to get rid of these spots but if they're too small you might want to select them first ie. with the magic wand tool and then you can Ctrl X them out of the picture to clean it all up.

Now find another picture which you think would make a great background. Now that you have the foreground subject and you've essentially removed the background, you can pull in another open image with the moVe tool by dragging it in.

It will automatically appear above whatever layer is selected in the layers palette. In this case it shows up on top (because the previous top layer was highlighted)

so you will have to move its order in the layers palette to beneath the main subject layer to ‘fit' in the manually created ‘background' space.

In this case the sunset was appear behind the main subject so if this happens to you remember that you have your transform options under the Edit menu. Go ahead and flip it horizontally.

Now the sunset appears on the other side where there is breathing room for it to be. This is good common design sense which you probably have, it's just good to know the tools to how to do it!

Get my DVD Basic Photoshop Tutorials here for less than 50 bucks.