Originally, it is theorized that comets, which are made up almost completely from ice, crashed into the Earth during the "great bombardment" some 4 billion years ago. The water we have today is the same amount we have always had, through the cycle of evaporation and rain it is constantly recycled and redistributed throughout the world. The water you use most likely comes from your local Water department.
From the oceans or bodies of water. Rain doesn't just come from space, when the water from oceans or lakes or seas, etc, evaporate, t turns into a gas (water vapor) and that's what clouds are. Eventually the water vapor turns into a liquid, and falls down to the earth in a sort of forcing fashion, otherwise know as rain. As the rain water falls back into the ocean, it repeats the same process over and over and over and over and over and over and over and over and over and over, and over again.