If your New to RPG Maker overall, like others have said, I definitely recommend making your first project plugin free and making a small game to learn what you can do with just the base editor and the event system down the line when you are comfortable working with the RPG maker engine and its editor, Look into Yanfly Engine, or Victor Engine, ect, and check out youtube videos for them, and see what you think best fits the game you want to make, and your personal development style. There are tonnes of videos on how to use the individual plugins. on top of that, there are a lot of good quality of life plugins from guys like SumRndmDde that work alongside the engines nicely.
As for 'Must have' plugins, there really is no answer for that, as the must have plugins really depend on what your game needs, to do the things you want it to do. If you go with yanfly engine plugins, Don't just drop them all in all at once, there's a LOT of them there, start small, and work your way up, These plugins have an order to them for installation too, if you have the plugins in the wrong order you can get a lot of errors. A really big quality of life thing you can do to get the most out of yanfly plugins is join his ******* at at least the $5 level, and get access to his Sample project, that has all of his plugins in it and setup with a lot of example setups that you can play with. Loading up his project into the editor along side your own game firing up the sample project in test play and going into the 'royal library' where all the example setups are, you can see how they run, and get descriptions of the plugins and the like, and then look at the areas in the editor that deal with those plugins, you can learn a LOT from rooting around in there.
Shilling aside, Adding plugins as you need them, really is the best way to go. In that line, expanding from the core to the extensions ect. But seriously, start small and don't overwhelm yourself.