Depends what you want to do with your team. If you want to play by Nintendo's rules in the Battle Spot or Tournaments, you will need to think a lot differently than if you are building a team for OU, the format most people prefer to play on. And in official stuff you will need to soft reset for 2 legendaries, but that remains beside the point.
The best place to start is inherently Smogon, as it offers competitive analyses on each pokemon that matters. Then, I
STRONGLY recommend building a team on Pokemon Showdown before building it in-game. This allows you to test your team with minimal effort, and see what others are running.
Here are some links to get you started:
http://www.smogon.com/dex/xy/pokemon/ - THE starting point and teambuilding resource.
http://pokecommunity.psim.us/ - PokeCommunity's Showdown server. There will almost always be someone willing to help you test and improve your teams here.
http://www.smogon.com/dex/xy/formats/ou/ - Description of the current OU format, aka the one people use outside of official tournaments and the Battle Spot. Also has a number of useful links, like sample teams and the viability rankings.