Poker is one of the most in-depth card games in the world. Bridge is the only game I know of comparable in difficulty and strategy. I don't play bridge myself, but I've heard some good things from gold-card-holding seniors at the Legion.
Poker being as in-depth as it is, it's ludicrous to think you could jump into the game and play it with any sort of competence without at least a moderate understanding of the theory and rules of play. To best explain what I'm talking about, I'll use a race car analogy:
You could technically climb in the window of a Nascar car and race the Indy 500. The gas pedal is still on the right, brake on the left. But without significant driver training, study and practice, you will never come close to winning. In fact, you would have to be lucky just to finish the race.
Just because you know how to play Gin, Crazy Eights and Go Fish doesn't mean you can learn guide on the rules and betting structure, and you should join an online poker site to play a bunch of free hands.
Once you know how the game works, and have an idea of what hands to play, and how to play them, you'll be able to sit in real games and start to learn the more intricate strategies and theories of poker.