Water is a self sustaining resource in that it can create itself through a cycle called "the water cycle". Of course, that doesn't answer the bigger question of "where does water come from?" This question was posed on the internet and so far the best answer has been that oxygen is the most common element of the earth's crust and hydrogen is the most common element of the universe, so it is almost inevitable that the two molecules would find each other and form bonds. This answer was posed by Pendour on Yahoo answers and so far as been chosen the Best Answer of the bunch.
So, where does water come from? In addition to Pendour (the earth's oxygen combining with the universe's water)'s answer, the answer could be that water comes from water. You obviously remember the water cycle you learned in elementary school: the water in the lakes and oceans is evaporated by the heat of the sun and forms a vapour that joins together to form clouds. When the temperature is warm enough, the vapour condenses into drops of water which is what rain is made out of. The rain falls onto the earth-onto land as well as back into oceans, lakes and rivers. From the water bodies it evaporates and the cycle starts all over. The water that is found on the earth is used by plants and often people as the water will eventually find its way to the water table.
What about your drinking water? Where does that water come from? Drinking water can come from two places-ground water and surface water.
Ground water is usually stored in aquifers that are underground areas where the rocks are very porous, like sponges. Water is taken out of the aquifers by wells and pumps that get drilled into the ground. Ground water is often thought to be superior to surface water because it is already mostly clean. The ground's soil acts as a water cleaner and filter as the water seeps through it and down to the aquifers. If the water contains chemicals or waste, most of those will get absorbed into the soil.
Surface water is the water that is found in lakes, streams, wetlands and ponds. These waters often carry a certain amount of salinity (salt seeping into the water from the oceans). Surface water needs treatment before it can be safe for drinking and everyday use. Surface water is passed through water treatment facilities that are built specifically to clean and filter water and then pumped out to homes and businesses. The treatment facility acts much in the same way that the soil does only in the treatment facility; the water is filtered and "enhanced" by machines.
Answering a question like "where does water come from" can be as easy as you'd like to make it. It can be as simple as water comes from rain all the way up to the explanation of how oxygen and hydrogen bond together to create individual water molecules.