You haven't posted a link to the specifications for the RF transmitter and receiver.
What Arduino are you using?
You can't expect to receive data on the Serial connection from your RF device and to send data to the PC with the same Serial connection. An Uno has only one hardware serial connection (pins 0 and 1) so you could use SoftwareSerial to create another serial connection on two other pins. Use SoftwareSerial for your RF device. How to use SoftwareSerial is in the Reference section.
...R