Merchant Accounts and Gateways

Could someone talk me through setting this up for the merchant account and payment gateways? What files do I need to modify and so on?

Are you referring to creating a new integration with a specific payment gateway or using one of the gateways already built into nopCommerce. If it is the latter, which gateway are you wanting to use?

The client is using TMC as their merchant and they want to use Paytrace as their gateway.