As far as new game mechanics go, as furret said, there is the addition of fairy types in the metagame. A lot of them are older pokemon that either lost a type in place of a new fairy type, or they just went from normal to fairy. They are pretty strong in resistances, however, it is best to counter them with either steel or poison types. Avoid using a dragon type against them since they resist it. I assume you are familiar with the concept of mega evolution? If not, it's essentially activated by a mega stone you place on your pokemon that allows you to mega evolve it (with the mega ring or anklet). With this "evolution" your pokemon will be able to reach new levels of power. It is crucial to become familiar with mega pokemon as they are a prominent part of the competitive metagame in X an Y.
When making an Uber or OU team, it is essential that you make a well rounded team, both in typing, and in role/purpose. You probably already know this, but I'll go over it anyway. Each pokemon has its own role, for instance, Ferrothorn serves its role best as a set up pokemon. It is best to have it as your first pokemon because it can set up status and spikes/stones on the opposing team early in the battle. Mega Garchomp however would serve as a sweeper because of its high speed and attack stats. It is best to use it early to mid game for best performance.
When it comes to starters, chespin becomes more defensive, froakie becomes a fast/special attacker and fennekin also becomes a more special attack based pokemon. Not to sway you in any way, but I chose greninja for a few reasons. 1) I liked his design, 2) his ability protean (which allows it to change to the type of the move it last used) sounded promising. 3) I was excited to potentially have a speedy water type for a change. Like I said, it's your choice, they all have their specialties and serve their own purpose. So choose whichever you want.
That about covers it as far as what you'll need to know. I would check smogon if you are looking for more information regarding competitive battling in X and Y. If you've ever competitively battled in prior generations, then most of it will seem common sense. There aren't really a whole lot of changes with the exception of fairies and mega pokemon.
I hope you found this to be somewhat helpful. I wish you the best in X and Y!
As far as new game mechanics go, as furret said, there is the addition of fairy types in the metagame. A lot of them are older pokemon that either lost a type in place of a new fairy type, or they just went from normal to fairy. They are pretty strong in resistances, however, it is best to counter them with either steel or poison types. Avoid using a dragon type against them since they resist it. I assume you are familiar with the concept of mega evolution? If not, it's essentially activated by a mega stone you place on your pokemon that allows you to mega evolve it (with the mega ring or anklet). With this "evolution" your pokemon will be able to reach new levels of power. It is crucial to become familiar with mega pokemon as they are a prominent part of the competitive metagame in X an Y.
When making an Uber or OU team, it is essential that you make a well rounded team, both in typing, and in role/purpose. You probably already know this, but I'll go over it anyway. Each pokemon has its own role, for instance, Ferrothorn serves its role best as a set up pokemon. It is best to have it as your first pokemon because it can set up status and spikes/stones on the opposing team early in the battle. Mega Garchomp however would serve as a sweeper because of its high speed and attack stats. It is best to use it early to mid game for best performance.
When it comes to starters, chespin becomes more defensive, froakie becomes a fast/special attacker and fennekin also becomes a more special attack based pokemon. Not to sway you in any way, but I chose greninja for a few reasons. 1) I liked his design, 2) his ability protean (which allows it to change to the type of the move it last used) sounded promising. 3) I was excited to potentially have a speedy water type for a change. Like I said, it's your choice, they all have their specialties and serve their own purpose. So choose whichever you want.
That about covers it as far as what you'll need to know. I would check smogon if you are looking for more information regarding competitive battling in X and Y. If you've ever competitively battled in prior generations, then most of it will seem common sense. There aren't really a whole lot of changes with the exception of fairies and mega pokemon.
I hope you found this to be somewhat helpful. I wish you the best in X and Y!