In this tutorial I'll show you how to detach an object from photo and create a new image with a different background. I'll use the large platinum ring in this photo as an example. This tutorial should work with all versions of Adobe Photoshop.
1.) Click on the POLYGONAL LASSO tool on the toolbar. Note the descriptive yellow box that appears if you hover over the tool for a few seconds. If you click and hold, more lasso tools are available, but we'll use the polygonal lasso for this demonstration.
2.) Position the lasso at your starting point and click the mouse and pull the lasso. As you make turns, you must click again and again and stive for a smooth curve. Each time you click you will be adding an anchor point and then pulling the lasso from that point. Look closely to see the lasso in the photo.
3.) Once you've gone all the way around, a small circle will appear near the lasso indicating that you're near your starting point. Once you click to complete the circle, the lasso becomes a selection. Selections in Photoshop are indicated by an animated dotted line of 'marching ants'.
4.) We must now duplicate the layer our rings are on by right-clicking its layer in the layers panel and selecting DUPLICATE LAYER. Photoshop limits editing of Background layers, so we'll be editing the copy. Once duplicated, click on the eye icon to the left of the background layer to turn it off. Click on the background copy layer so it becomes the active layer, then proceed to the next step.
5.) Now open the SELECT menu and click on INVERSE. This inverses the selection, meaning instead of the ring being selected, everything BUT the ring is selected (its background). Note that the selection is also along the parimeter of the image as opposed to being only around the ring as in the previous step.
6.) Press the Delete key and everything but the ring disappears. Photoshop indicates a transparent area with a grey checkerboard background.
7.) In the menu at the top of the page, go to LAYER>NEW>LAYER and add layer BELOW the ring layer. Layer order is easy to change by clicking and dragging up or down in the layers panel. Using the eyedropper tool and the SWATCHES panel, I selected a dark blue for the background and make sure my new layer is the active one by clicking on it. Going to EDIT>FILL>FOREGROUND COLOR (Option+Delete on a Mac), the new layer is filled with my dark blue color.