You mean the subject is in different enough parts of the frame so that they don't overlap at all, right? Ok, this is pretty easy, but you've gotta know the basics. Pick any image, just a regular photo from the Web or wherever. Just so you can learn these basics.
Start with the Rectangular Selection Tool, aka the Marquee Tool, keyboard shortcut 'm', which is 2nd from the top of the tools palette and looks like a rectangle with a dashed line. Drag it around part of the image to make a rectangular selection. Edit->Copy, Edit->Paste. (Or Ctrl-C, Ctrl-V.) You've now got the selected part of the image copied and pasted. When you paste, Photoshop automatically creates a new layer on top of the old one. So you can move the pasted area around (using the Move tool, an arrow, top of the Tools palette, keyboard shortcut 'v') without changing the underlying image. All the layers in the image are shown on the Layers palette, and you switch between working layers by clicking on them in the Layers palette.
Now, if the selection needs to be more, well, selective than a simple rectangle, you've got a lot more work ahead of you. You can use the Lasso Tool (freehand selection), the Polygon Lasso Tool (freehand selection with straight lines), and the Magic Wand Tool (selection based on color similarities, based on a numeric sensitivity that you can change in the toolbar). This can get very tedious, as precision is hard to achieve. You can add new selection to a previously-selected area using Ctrl and Shift modifiers.