If you want to obtain Reshiram, you will need to trade with someone who has a copy of Pokemon White and has captured Reshiram.
Here's how you can get Reshiram in Pokemon Black via trading:
1. Find a trading partner: Find someone with Pokemon White who has captured Reshiram and is willing to trade it. You can find trading partners online or in person.
2. Agree on the trade: Discuss what you will trade in return for Reshiram. This can be another Pokemon, items, or something else you both agree on.
3. Connect your systems: Use the wireless connection feature in Pokemon Black to connect to your trading partner's system.
4. Initiate the trade: Both of you will need to select "Trade" from the menu.
5. Complete the exchange: Once you've both selected the Pokemon you want to trade, confirm the exchange.
Remember, you will need to have access to Pokemon White in order to trade for Reshiram. You cannot capture it in Pokemon Black.