Does anyone know how to know what Phoentic Letter to say when trainsitioning frequencies? I don’t know what to say. Also, when filing a flight plan, do you have to enter the flight plan into the MCDU, or can you just fly without it?
I am not really sure what you are asking here, could you try rephrasing or elaborating on your question?
You have to be able to fly what you are cleared for. Having the cleared route in your FMC usually makes that significantly more easy 
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I guess you mean how to call the next controller when instructed to switch frequencies?
You usually have just one Callsign throughout your whole flight.
E.g.: If I file a flight with callsign TUI1708 I connect to VATSIM using TUI1708 and I say this callsign every time I have contact with ATC. No matter if it’s Delivery, Gound, Tower, Radar or Approach etc.
When GND instructs me “Tuijet 708 contact Tower 119,4” I acknowledge by saying “TWR on 119,4, Tuijet 1708”.
Then I switch to TWR Freq. 119,4, listen out on the freq for a couple seconds to make sure I don’t interrupt an ongoing conversation or readback and then I call in “Tower, hello, Tuijet 1708, holdingpoint A3, RWY29, ready for departure”.
If that’s what you mean 
Edit.: watch some VATSIM Streams on Youtube and you’ll get a good idea of what to say when quite quick.
The phonetic alphabet can be found here: ICAO Phonetic Alphabet | SKYbrary Aviation Safety
The callsign of different airlines can be found via https://www.avcodes.co.uk/airlcodesearch.asp or google.
Best advise regarding flightplans into flightcomputer (MCDU or FMC): Use it if you got it = learn you plane. Otherwise fly a less complicated airplane.
Thanks! I know now that the phoentic word to say is based on the atis, so I just have to listen on the atis before talking to ATC when entering their airspace. Cheers!
Years ago,in the days of Q Codes , (based on the Morse Telegraphy communication not speech),there was one which meant “Changing Frequency” and it was “QSY”.
We still use QFE,QDM,QNH ,QNE,QTE etc, spoken over the radio.
Some people used the expression QSY too: “GBETC QSY to Tower on 113.7” for example or simply “GBETC QSY 113.7”.