First of all you have to find N in N's Castle, a Zoroark will guide you there. You will battle N. When you beat him Zekrom will turn into the Dark Stone and you have to take it to Dragonspiral Tower. Catch Zekrom and then go to Giant Chasm. Kyurem will be there.
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You have to go find Kyruem; after you beat Ghestis. You should lower Kyruem's HP before capturing. You should have a Master Ball, that would be the best way.
you have to beat the elite four
You can fight Cynthia in black or white after you beat the elite four and ask for him/her at GTS.
You just get legendary Pokemon I guess
acually there is but not Reshiram or Zecrom but you can get cobalion terrakion and virizion