It depends upon exactly what you want to copy & paste. (It might also depend upon where you want to do it. My instructions below relate to Windows-based computers. Android tablets, etc, might be different).
If, for example, you want to copy text from another website to a post here, go to the relevant website and drag your cursor across the relevant piece of text (while keeping your left mouse button held down). That will highlight the text. Then right-click on the highlighted text and select 'Copy'. Come to the AB post where you want to paste the text, right-click where you want the text to appear and click 'Paste'.
(Note: Pressing Ctrl and C together also tells your computer to copy selected text. Pressing Ctrl and V together tells it to paste it).
If you want to copy and paste a link here, you go the same way about it. i.e. go to the web page that you want to link to. Highlight the URL in your browser's address bar. (Right-clicking on it suffices). Then select 'Copy', come to an AB post, right-click and select 'Paste'.
If you want to copy and paste files from one folder to another on your computer, open a window showing the first folder, right-click on the relevant file and select 'Copy'. Then open a window showing the second folder, right-click inside it and select 'Paste'. (Note that you can select an entire block of files for copying, by clicking on the first one and then holding Shift down while clicking on the last one. To select lots of individual files at the same time, hold Ctrl down while clicking on them one by one).