When an earthquake, volcano or landslide happens on the ocean floor, water is displaced. This water forms the start of the tsunami.
When the waves reach shallower water:
It is hard to see that a tsunami is approaching. The most obvious sign is the coastal water retreats just before the waves reach the shore. This is actually the trough of the wave following behind.
The main impact a tsunami has is flooding. The waters are also able to erode the foundations of coastal structures.