Photoshop Selections: The Elliptical Marquee Tool
By default, the Elliptical Marquee Tool is hiding behind the Rectangular Marquee Tool in the Tools panel. To access it, simply click on the Rectangular Marquee Tool, then hold your mouse button down for a second or two until a fly-out menu appears showing you the other tools that are nested behind it. Click on the Elliptical Marquee Tool in the fly-out menu to select it:
Once you've chosen the Elliptical Marquee Tool, it will appear in place of the Rectangular Marquee Tool in the Tools panel. To get back to the Rectangular Marquee Tool, you'll need to click and hold on the Elliptical Marquee Tool, then select the Rectangular Marquee Tool from the fly-out menu:
Whichever Marquee Tool you selected previously appears in the Tools panel.
You can easily switch between the Rectangular and Elliptical Marquee Tools from your keyboard, although exactly how you do it will depend on how you have things set up in Photoshop's Preferences. Every tool in the Tools panel can be accessed by pressing a certain letter on your keyboard. Both the Rectangular and Elliptical Marquee Tools can be selected by pressing the letter M, and to switch between them, you either press M again by itself or you'll need to press Shift+M. Again, this depends on how you have it set up in the Preferences.
On a PC, you'll find the Preferences option under the Edit menu at the top of the screen. On a Mac, you'll find it under the Photoshop menu. In the General section, look for the option called Use Shift Key for Tool Switch. With this option checked, you'll need to add the Shift key to switch between the two Marquee tools (as well as other tools in the Tools panel that share the same keyboard shortcut). Uncheck the option if you'd rather just use the M key by itself to switch between them. It's completely up to you: