When you are ready to render an image, select Render > Render Image from the main menu at the top of the screen. This opens the Render Image dialog.
In the Render Image dialog, check (enable) the Render Alpha Channel option. This will render only your object(s), with an alpha channel. An alpha channel is a single 8-bit (grayscale) channel in a color image used to store transparency data. The alpha channel makes it easy for you to place renderings into image and animation composition applications.
You may also want to check (enable) the Exclude Background option. See Chapter 18 - The Render Image Dialog > Alpha Channel and Background Settings, on page 353 in the User Guide. The User Guide is available in the Help menu.