Select the text or item you want to copy
Press Command (⌘) + C to copy
Move the cursor to where you want to paste
Press Command (⌘) + V to paste
To cut instead of copy, select the item and press Command (⌘) + X
To paste without formatting, use Option (⌥) + Shift + Command (⌘) + V
To copy from a screenshot, press Command (⌘) + Shift + 4, select the area, then paste with Command (⌘) + V