Arduino Interface with Serial device and LCD display
If the device is serial and you want to connect it to the Arduino serial port, why would you use a usb-to-serial converter?
For a serial to serial connection, you have to connect TX, RX and GND.
This is the bare minimum for bidirectional communication, if your device is really read only, you can use just the TX and GND.
it depends how you receive data from your "serial device" - btw, what is it? If the data comes as either ascii or individual characters, then it's easy: treat them as strings and display them separately. If the numbers arrive in binary format, you have to perform the conversion to string.


Questions like this should have attached at least a snippet, if not all, of code.