On your mouse you should be able to click left (with pointer finger if right handed) and right (with middle finger if right handed). Move your mouse to the left of what you want to highlight (for instance, if you want to highlight this post you would put your mouse right next to the first word "On"), now left click and hold it down as you move the mouse so it highlights whatever you want to copy.
Now let go of the mouse. The text should stay highlighted.
Now right click over the highlighted area and choose "copy".
Now go to wherever you want to put that text and right click and chose "paste" and it should magically appear.
Did it work?