The leftmost column of each track contains the track number. This column is also used for selecting tracks. Left-click some track numbers (for best results, do this with tracks you've loaded audio into). Notice that the tracks change color. This means the tracks are selected. Press Ctrl+C, and then Ctrl+V. You've just copied and pasted (cloned) the tracks.
When the tracks you want to select form a contiguous group, instead of left-clicking them one at a time, it's easier to use one of these methods:
To clear the current selection, press Esc, or choose Deselect from the Edit menu.
Notice that one of the track numbers is always a different color (red). This indicates the current track. When you modify a track (e.g. by adjusting one of its sliders), it becomes the current track. You can also make a track current without modifying anything, by left-clicking on its name.
In general, Edit and Track menu commands operate on either the selected tracks, or if no tracks are selected, on the current track.