What are the differences between the various editions of Pathfinder?
What class is good for a beginner?
Currently I play Neverwinter Night 2 but I think Pathfinder is ver different, which class is best for a beginner?

It's 1st edition PF.
Most classes are viable and it is very different experience from NWN2. A lot more hardcore.
For suggestions it would be helpful if you would provide what you want from your MC.

I think there's officially only one edition. Pathfinder 2 is in a play testing phase.

What I mean is: Royal Edition, Explorer Edition, Imperial Edition? What are the differences?

No problem. :-)
Do you have an idea what class is good for a beginner?
Maybe a Fighter > Tower Shield Specialist > Stalwart Defender?
By the way, what is the max level a character can reach?

Technically, you can reach 20 and it's possible to do so. Now if you would ask what level most people end up as - it would be 16-18
You will have TSS/Stalwart companion, so there's that.
For a fightery type with something better to do I'd say any sort of vivisectionist is not a bad idea.

What about a Gnome/Sorecerer?
the female gnomes seem cute in this game, unlike NWN2 where they were ugly.

I got this from the store page: https://imgur.com/a/ngcqZ4g. I only have the Explorer Edition because I'm a tavern wench. You also get a pet named Ember that gives +1 to morale saving throws. Here is his description: https://imgur.com/a/3y2byuM (info and image of ember taken from https://pathfinderkingmaker.gamepedia.com/Ember). I am not aware of any other differences that are not listed here.

sorcs are in a good place. draconic for shifters, sylvan for utility/ray, arcane for DC nuking. nomes though... they are ok, I guess, but in reality they are just bad halflings in this one.

Both @InEffect and @Roahin have good threads on companion builds which will give you an idea of what the classes can do. InEffect's also includes some mainclass builds along with general build notes.

Thanks for the links, very helpful. :-)