To copy and paste on a PC or Mac, highlight the text by clicking and dragging across it. Then, go to Edit -> Copy from the menu bar at the top of the screen. Then, once you have a blinking cursor wherever you'd like to paste the text, go to Edit -> Paste from the menu bar.
To copy and paste on an iPad or iPhone, double-tap some text to highlight it. Drag the blue bars to the beginning and end of the text you'd like to copy. There should be a popup with some options - tap Copy. Then, once you have a blinking cursor wherever you'd like to paste the text, tap the screen and choose Paste from the popup.