Consider the idea of an online world playing your favorite game of Pokemon - only you're not the only playing character around, its not just NPC's your talking to and it isn't merely computer generated battle scenes that you're up against but actual players with actual battle set skills, who are just as wily as you and probably thinking of the same strategy as you are.
Interact with other players and walk around the pokemon world you've come to familiarize yourself with and find that, well hey - this place is just as good by myself as it is with other people! Gameplay is basically almost the same with other pokemon games you've played before, meaning if you can still take out you Game Boy Color, load Pokemon Blue into the slot and not get bored with it - then this game is meant for you to play with!
You'll start the game off, as what most adept Pokemon players are familiar with - your hometown. Everything is pretty much basically like you were playing it on your DS. Start in your room, talk to your mom - get a Pokemon and rescue the professor. The big difference here is that well, you'll be seeing other players like you out in and about playing the game like you would play it.
So you really just play it as it is - begin your journey by talking to Professor Oak in the lab (as you normally do). He'll give you clues as to what you'll need to do next (which is rescuing a fellow professor from the clutches of a wild pokemon).
The basics is pretty much the same as how you would normally approach a Pokemon game, save the professor, get a pokemon, catch pokemon, get a pokedex. From there on out its really just a matter of exploring the world, getting Gym Badges and defeating the Elite Four.