If you get hired by a 121 operatior, you will have time in ground school and/or the sim sessions to learn the basics. You can pickup the more advance stuff while flying the line.
If you hired by a 135 oerator and they have internal training, you can typically learn "on the job" while at cruise. If you goto school (like FS or Simi, Pan Am, etc.), then you can pickup "how to" there.
It is really not that complex of a device if you can operator a computer. You just have to think in the lodgic that the engineers used to write the FMS software.
Just my 2 cents.