You go to the Pokemon center. Right next to the globe, there's a girl who asks you to do wireless communications. then say global trade
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Actually you will not be able to find a Groudon in pokemon black or white. It's because you will not be able to find a Groudon in the wild. The only way you can get Groudon in pokemon black and white you will have to trade with someone who has a Groudon or you can use Global trade.
Trade from white and if you have white trade from black for tornadous
Pokemon White, and other copies of Pokemon Black.
You have to trade a starter to another Pokemon white or black game then trade the Pokemon back.
Celebi isn't available in Pokemon Black and White. You have to transfer it into Pokemon Black and White or trade for one.