High in the Himalayan mountains of India and Nepal you can find natural water in its most wonderful and healthful state, not only delicious but so full of minerals and vitamins from glacial runoff that it can actually extend one's potential for life. Some of the oldest and healthiest of humans on this planet live nearby, drinking the water gifted by god and eating vegetables grown from this rich water.
As this highest of natural waters runs into rivers and passes hundreds of miles of potential pollution of its life-giving energy, the natural form of purification is still the best. In Rishikesh for example is a simple natural well deep in the earth. From this well you can hand pump some of the most delicious and healthy water you could ever imagine. It still contains its life force derived from glacial waters at the source of the Ganga river, hundreds of miles away, but the natural filtering of rocks beside the river remove the potentially harmful influences of its travel, leaving it cold, clean, and delicious. I have personally experienced this water, by its own nature, removing tiredness, dullness, even sickness, almost instantly, and leaving me feeling as if I had tasted a drink from the heavens.
All over the world there are natural springs and wells that have water provided by the earth and by God in a pure and life giving form. But if you are not near one of these places, then you can copy the earth by filtering in a similar way. There are special filter stones available that simulate the filtering provided by the earth, removing all the harmful bacteria and other things that can get into water, keeping most of the good things.
If you feel you must resort to less natural means, then you can purchase reverse osmosis filters almost anywhere in western countries. Ideally, this is a porous membrane with spaces only large enough for the water molecule to fit through, removing everything else. Unfortunately, as it removes all the bad things it also removes the good and life giving minerals from the water. Thus, a modern water system will usually contain a carbon or other "filter" after the reverse osmosis filter, and that will add some minerals back in. Some of the best types of carbon filters are made from almond shells or from coconut shells. BUT BEWARE ! Some of the more unscrupulous suppliers use the rejected bones of dead animals such as cows and pigs to make the charcoal of a water filter, converting a very nice process into an ungodly one.
If you want something in between, or have known specific impurities that you want to remove from your water supply, then there are lots of special activated mineral filters that remove specific targeted minerals, metals such as lead or arsenic, or others. The normal activated carbon filters that you buy in the grocery store help some, but mostly just remove simple minerals and improve the taste of your water, sometimes even making it less healthy.