For this tutorial grab the lasso tool in the toolbar.

Now make a general selection by dragging the lasso around your subject.

Try to keep it as close as you can. You have to go all the way around the subject to meet endpoints to close off the selection.

This is a basic and old school way of making selections which you must understand. There are much 'better' ways which I cover in the Basic Photoshop training but once again; imperative that you understand the fundamentals and grow from there.

Once you have made a selection around your subject, switch to the moVe tool as shown.

Now you can place the cursor inside the selection and ‘move' it. Try doing this and dragging it around.  See how the space is left there where she used to be? 

If you're wondering where the actual water and sidewalk went then I can't help you (but yes you could fix it).

Go to the History palette and undo your last command of move to put the subject back in place.

To work with your new selection on it's own, go to Layer: New: Layer via copy. This places the selection onto it's own layer (very important to understand).

You can also right click when you are on a marquee or lasso tool and choose Layer via Copy to put the selection onto its own layer. There are ways to edit the (lasso) selection which are covered in Basic Photoshop training and other tutorials here.

Now you have basically duplicated your subject and put it onto its own layer. Here I've got her on her own layer (you can see the invisible space around her in her layer on the palette).

Remember that there are better, more accurate ways of selecting which we'll get to. It takes a steady hand to use the lasso.