Pokemon aired its first episode in 1997. The original series is referred to as "Pokemon Original" and is composed of the first 5 seasons. The series follows the adventures of Ash Ketchum and his friends through the Kanto and Johto regions.
You can watch pokemon episodes online right here at pokemonfire.com. Simply click the season you want to watch from the left sidebar and you will be presented with the episode list. Enjoy watching pokemon!