I would suggest to start on a small controlled airfield to learn about VATSIM. just make a couple of circuits, getting used to ferq changes and basic ATC trafic. Than start a cross country VFR flight and build it slowly up to IFR on large airports.
What I did was first spawn on a big airports apron and just listen to all the ATC chatter. Try to follow 1 aircraft and imagine you need to do the radio work for that particular flight. Its quite basic and repetitive once you get the hang of it. Make notes, watch youtube tutorials (there are a lot of them) and even listen to real ATC on LiveATC for example. I even start to talk ATC in the car at a certain moment.
Dont be afraid to make mistakes, but just get well prepared for a flight. know the basic terminology, get your charts ready and filea good flightplan. the rest is just practice and there are a lot of willing people out there to help or give advice.
Myself, I dont do Vatsim anymore because I like to relax during the flight and dont bother with ATC anymore. That doesnt make the flight far less realistic, because i really like to still navigate with the charts (navigraph) and flightplans from simbrief.
And with the release of the DC6, oh boy… thats back to old school navigation.