Watch the tutorial here: https://youtu.be/jry6Wti83nY
How to morph shapes:
First, create your composition. Then, create a solid layer.
With the solid layer selected, draw your shape. This will create a mask using the shape you've drawn. Repeat this step with your next shape. Now, the first and second shape will be "mask 1" and "mask 2" under that solid layer. Rename them if you wish.
Position your shapes on top of each other on the artboard.
Select both the masks (shapes) and open the dropdown using the arrow in the panel. Select "mask path." Now deselect everything.
Move the (keyframe stick thing?) to where you want your next keyframe to be. Copy the original "mask path" keyframe of your second shape to your first shape.
Once you've copied and pasted your second shape to the first one, you can delete the second shape. Voila! You're done.
But say you want to be smoother... Enable motion blur. (It's the 3 circles with 3 different shades of grey in the panel.) Then, change the keyframes to "easy ease." You can also do this with the F9 shortcut. Now you're done unless you want to do something fancy.