Useful websites

I use different websites when I am building a team. You might use different websites and that is okay, but this is my personal preference.

www.serebii.net 

Some like to use Bulbapedia others Serebii, its all a personal preference. On these websites you will find a Pokedex with all the Pokemon you can find in the game. How to evolve them, what moves they get at what level, where you can find items, how to use them etc.

http://www.smogon.com/dex/xy/pokemon

I like to use the Smogon strategy dex as a reference when building a Pokemon. Smogon is a community that has their own set of rules and a lot of people like to use their rules. (I like VGC rules more)

http://play.pokemonshowdown.com/teambuilder

Pokemon Showdown has a great feature if you need a place to build a Pokemon. Here you can add moves, items, EVs, IVs etc. You can install the software on your PC or use it online. They also run a damage calculator which is good to use when you battle competitively.  If you build your team they even have an engine you can use to battle using the Pokemon you created. 

http://www.marriland.com/tools/team-builder

I use Marilland Teambuilder to check the overall weakness overview of the Pokemon team I created. If you have a lot of the same type Pokemon one type move might cripple your team. So it is good to check the weaknesses to see how you will counter those in battle.