Screen Shots

If you want to create an image of anything on your screen, make a screen shot:

Hitting the "Prt Scr" button on your keyboard
In Photoshop, open a new file (choose File > New, and then click OK)
Paste into the new document (hit control-v)
As desired, crop the image (select the desired content with the Marquee Tool, then choose Image > Crop)
Save the document as a Photoshop document (File > Save), or as a .jpg or .gif to use online (File > Save for Web)

Hitting Command+Shift+Control+4
Using the cursor to drag diagonally across the screen to select the portion you want in your screen shot
Hit "Enter" (a image file will be saved to the Mac's clipboard)
In Photoshop, open an existing image document (File > Open) or a new file (File > New)
Paste the screen shot image into the Photoshop file's document window (Command+V)
As desired, crop the image further (select the desired content with the Marquee Tool, then choose Image > Crop)
Save the document as a Photoshop document (File > Save) for further editing, or as a .jpg or .gif to use online (File > Save for Web)