Do look up these website for care of bettas:
http://www.plakatthai.com/index.html
http://www.bettysplendens.com/articles/home.asp

There is a difference in distilled and purified water. In the layman’s point of view, distilled water is very very pure water (i.e. contains no solid impurities, and little, if any electrolytes = minerals, salts etc), made through distillation, where the water is boiled and the condensed steam collected to result in distilled water (in rough sense). This is not so suitable for aquatic life to thrive in.

Purified water is water where the more solid components of impurities, like bacteria, silt, etc has been removed, but leaving the mineral content untouched. One example of purified water is water purified through Reversed Osmosis. It is very pure, but not up to that required by science labs. Bottled water should be ok for use. If you’re not sure, then add in some anti-chlorine or water conditioner, just as Dom said.

As Dom mentioned, use only aquatic plants, and not land plants. Some examples are Elodea, water hyacinths, water lettuce, water ferns etc. Do note that some of these plants are classed as aquatic weeds, and so be careful when and where you dispose excess plant materials. Never flush then down the drain, or release them into water ways. Turn them into mulch for your garden instead.

Water changes depend on the situation. Generally, once every 4-5 days would be best. Change the water as soon as it seems cloudy, or smells. If it smells, that is usually quite bad, meaning that the fish has been overfed, leaving uneaten food particles to rot and turn the water anaerobic (oxygen depleted).

The rest are as Dom said.