Take one step at a time. For example, if you are struggling with busy airspaces, you can try to familiarize yourself with the procedures there by flying outside of local evenings/weekends when traffic levels are usually considerably lower; you could also fly to less busy airports within an overall busier airspace which will usually make especially the critical, high-workload phases of your flight (e.g. the approach phase) much easier as you will likely be the only aircraft currently approaching a given airport. Once you feel more confident, you can step it up and for example try to depart from a busy airport but fly to a calm one and so on.
This way, you can reasonably ensure that you don’t add too many new things at the same time and efficiently debrief the new things you tried - if you then come to the conclusion that you did well, you can take the next step, or, if you felt that you still struggled, you can repeat the same step until you feel confident enough.
In general, it is also a good decision to not jump into fast and complex airliners that are designed to be operated by at least two people immediately and instead fly slow and non-complex general aviation aircraft as you will have much more time for everything and can concentrate on the complexity that VATSIM itself already adds.
In line with Andreas’ comment: to answer this question in a way that is applicable to your situation, we need to know more details. However, I have a few general observations.
For route planning, there are a few helpful tools: the GRD will show valid routes including level ranges, EDI-GLA has real world flight plans, and on SimBrief some routes are marked to show you that they have been validated (e.g. the RealWorld, Eurocontrol, or NATS icons, and there are even preferred routes that can be set by vACC staff - the VATSIM icon, however, does not show that a route is valid). What all of these tools have in common, however, is that you won’t find valid routes for every conceivable combination of airports.
If you really want to create “the most realistic and efficient flight plans possible”, you will probably have to put in the same work as real world dispatchers which - depending on where you fly - can be an incredibly complex task (in Europe, for example one of the most important documents for flight planning is the RAD which changes every AIRAC and is currently 640 pages long, written in relatively small text - then again, the validator provided by Eurocontrol (accessible via the Flight Planning section in the NOP) can help you identify issues with your flight plan relatively quickly but solving them may still be a rather complex task).
Now, this is of course all talking IFR. VFR flight planning is usually significantly easier as you tend to be able to do whatever you want as long as you remain outside of protected airspace, but there are also usually more differences between different countries that you would need to familiarize yourself with.
For fuel calculations, SimBrief is probably the best available tool, particularly if you don’t fly really niche aircraft that don’t currently have a performance profile on SimBrief. Just yesterday, they even updated their dispatch tool with a takeoff and landing performance calculator.
But again, this is primarily a tool for IFR; it’s fuel calculation tools can of course also be used for VFR, but you may also be interested in LNM where you can also create or download basic performance profiles for your aircraft.
Weather considerations are a bit of a moot point in flight simulation in general, but here, once again, SimBrief can be helpful as the briefings it creates come with weather data and charts. If you own a Navigraph Ultimate subscription, their Charts app also has a large amount of weather data that you can look at while planning your flight.
Once more, more information, as already said by Andreas, would be really helpful. We don’t know what type of aircraft you are looking for (are you interested in single engine pistons for VFR flying, are you looking for modern jetliners that you want to take on long-haul flights, or do you look for something else entirely?) and of course what sim you are using.
Many addon developers nowadays just seem to slap “study level” on everything they make because it seems to make it much more desirable to the flight sim community, even if there is no real system depth except for the ability to flick every switch. However, it seems like everyone agrees that by far the most advanced, immersive, and in-depth aircraft addon available in flight sim is the Hotstart Challenger 650 for X-Plane, but there is a large variety of aircraft addons throughout all sims and flight that exhibit a pretty good system-depth.
The same as stated initially applies: take small steps. Of course it is expected that you know your aircraft well before taking it to VATSIM, so with more complex addons, you will likely have to spend considerable time flying offline practicing various procedures etc. before flying online. And during your first online flights, you should try to pick calm routes to avoid causing problems should you still come across something that you need to look up in the addon’s documentation or if something doesn’t work as expected.