First build a dike around the area which you want to claim from the sea. Then pump the water away threw canals. In the early days we used windmills to pump the water, nowadays we use electric...
they reclaim land from the sea so that they have more land to build on and so that it makes japan flatter with the land its using to reclaim the land which also gives them more space to build on.
Because the land mass of their country is small, reclaiming land from the sea increases the area available for agriculture, although it is on ongoing project to prevent the sea from taking it back...