Do you have to use a real flights flight plan?

Do you need to use a real flights flight plan or can you create your own? Also, what is the best free software to create flightplans (A lot of people use simbrief because its easy to import the flight plan into the A32NX fly-by-wire).

You do not have to use real flights fllightplans; you can create yourown. And simbrief is one of the best free softwares available.
But especially around airports the use of real world procedure, like SIDs and STARs and other standard procedures, the controllers very much would like you to use these procedures, as the procedures makes atc’s live much easier. When there is a lot of traffic there is little time to accomodate individual “strange” routes, whic can disrupt the traffic flow in the area. Enroute there are usually no restrictions. But as we try to make this a simulation of real life I strive to adhere to how real world routes are made. In Europe there are a lot more restrictions than in the US, if you want to make the simulation as real as possible, but it is up to you.