In order to copy or cut using a keyboard, you first have to make sure the content you want to use is selected. For visual elements like images or video clips, you'll likely need to click on them to select them, or, if you're using an editing program, you can use that program's selection tools. For text, you can either click and drag your mouse over the section you need to highlight or hold Shift (⇧) and use the arrow keys to manually select what you want.
Once selected, press Command (⌘) and C to copy the content, or Command (⌘) and X to cut it.
With your content now in the clipboard, navigate to the spot where you want to paste it. For text, you can either click or use the arrow keys to place the text cursor when you want the clipped content to go, or you can use the same methods outlined above to highlight a section you intend to replace.
When you're ready, press Command (⌘) and V to paste your content, or Command (⌘) and Shift (⇧) with V to "Paste and Match Style." Keep in mind the keyboard commands for the latter can sometimes differ depending on the program — for example, Pages requires you to press Command (⌘) and Shift (⇧) along with Option (⌥) and V.