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check what serial exactly you're dealing with. If it's e.g. RS485 it's current driven, and you need to deal with it through e.g. a MAX485 chip. RS232 is voltage driven, typically around 10V so also that needs a translator for that. There are many different serial protocols, which are not compatible.
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find out what serial data protocol is used. Start/stop bits etc. That you need to know to be able to properly communicate with the device.
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if speeds are low (<9600 bps), look into SoftwareSerial, this can turn any pair of pins into TX/RX, and you may be able to use a cheaper Uno rather than the Mega.