The Serial object (and also Serial2, 3 and 4 on the Mega) use the Receive interrupt to put the characters into a ring buffer... to get them is easy..
The Serial objects are well documented in the Reference section...
This code is as simple as i could make it.... assuming you use the 2nd Uart...
void setup()
{
Serial2.begin(9600); // use this port and speed for device
}
void loop()
{
char in_byte ;
if (Serial2.available())
in_byte = Serial2.read() ;
}