Hi Alice, you're new, so don't let this phase you. BartMan's advice is spot on, but what he doesn't mention is how simple MC makes this task for you, especially as you have them already neatly organised in folders.
The tool you are looking for is called "Fill properties from filename". You find that by selecting a file, or files, then right click, Library Tools, Fill properties from filename.
It's pretty self explanatory, but to offer a really simplistic example, pick a file...
Say the path was X:\Music\Genre\Artist\Album\Track # - Name.mp3
You could try the 'Automatic' option, notice the interpretation is shown on the right... and if that's not cutting it for you, fill your templates using the path and/or filename fields below...
In 'Path', you would enter: [Genre]\[]\[]
and on the right, you should see that MC will update the "Genre" tag for the selected file, and if the 'Filename' field is left unselected, this would be the only change applied when you commit the change.
Try it on a single file or two untill you are comfortable with the tool, then experience the wholesale joy of the tool by listing all files, selecting them all, and applying the fill properties tool across the lot in one go.
The only requirement is that all the selected files fit the template you are asking MC to use to fill the properties from, otherwise you could get some unwanted results.
If you need any further help, just ask.
-marko