There are many manuals on how to create skeletons for our characters. However, we present a basic example that you yourself must develop further, with better complexity, for each type of character you need to model. In this case, we will move our whale's tail, which is translated to the axis rotated in the y value.
Select Edit Mode to click on the last joint and press E to add another joint. Another way to do this is to go to the 3D View menu and select Armature | Extrude.