There is no single tutorial spot for all online trading card games. However, most online trading card games come with built-in tutorials and advice, offer precalculated decks and support fan communities that offer tutorials to new players in that particular game.
The competitive yet cooperative nature of trading card games, also known as collectible card games, means that fans are always keen to attract new players to joining them in their hobby, whether online or in real life. This means that the most likely place for new players to find support in learning whichever CCG they have chosen is the fan forums.
Most online CCGs, and indeed, most real-world CCGs offer support for a huge community of fans who help new players and guide them towards resources for learning the game. However, the rules and strategies for one game are unlikely to carry directly into another system, so there is little generalized support for new players who do not know which game they wish to play.