I want to know what are the differences between version X and Y, like pokemons, places, items, megaevolutions, etc.
Pokemon Exclusive to Version X
Pokemon Exclusive to Version Y
Source - Serebii
It's also been noted that there are minor story differences:
Source: Shack News
As always the main thing you should consider when choosing between Pokemon X and Pokemon Y is which version exclusive Pokemon appeal to you the most.
The main Difference are the 2 main legendaries, Xerneas for X and Yveltal for Y. There are also differnt mega evolutions for certain ones, like Mega Charizard X, Mega Mewtwo X and Mega Tyranitar (to name a few) for only X, while Mega Charizard Y, Mega Mewtwo Y, and Mega Gengar(again, just to name a few) only for Y. Hope this helps. Oh, i have some more. Mega Aggron, Swirlix, Slurpuff can only be found in pokemon X. Spritzee and Aromatisse can only be found on Pokemon Y.