Question 1:
http://www.smogon.com/bw/pokemon/ select the pokemon and it will give you at last 1 example build.
http://stats.pokemon-online.eu select the format and pokemon, it will show you the most used builds.
Question 2: You can always go to a stadistic page or something like that (like the ones I just linked you) and check how most people are building their pokemons. With enough knowledge over the metagame you will most likely know what most of your opponents pokemons's evs and natures are. Nature and Scarf are a bit harder to predict tho, specially Scarf.
Question 3: Previous gen OU pokemons are still the same. Weather related pokemons are still used (minus Politoed), but there are no longer weather based teams. You can start there.
In this gen, the most OU pokemons I have seen so far are Aegislash, Greninja, Mega-Gengar and Mega-Khangaskhan. Talonflame and Klefki are quite common. Goodra, Trevenant, Gourgeist and Diggersby see some play from time to time. The others only see play from in rare occasions. Megas are losing a lot of their initial popularity.