You can watch all episodes of Digimon season 1 for free at WatchDigimonEpisodes.com. In fact, you can watch every episode ever made of Digimon there. If for some reason you're unable to watch them there, they're often posted on YouTube, too.
Digimon began as digital pets known as Digital Monsters. They were created in a similar style to Tamagotchi's. You could train, evolve and have battles with other people's Digital Monsters.
In 1999, Digimon was created as a film in Japan, which led to the request that a TV show be created. This show would go on to become the well known Digimon Adventure, which was followed by a further five TV series and films.
As with other popular franchises, Digimon has its own trading card game, as well as a number of video games to enjoy.
Here's season 1, episode 1 of Digimon to watch - just in case you can't be bothered looking for it elsewhere!