Select the text or item you want to copy
Press Command (⌘) + C
Click in the destination (document, field, or folder)
Press Command (⌘) + V
To copy a file or folder: select it, then press Command (⌘) + C
To paste a file or folder: open the destination folder, then press Command (⌘) + V
To copy using the menu: Edit → Copy, then Edit → Paste
To copy a screenshot: press Command (⌘) + Shift + 4, select the area, then open the destination and press Command (⌘) + V