XnView Software

What is the best method to sort images?

That's up to you simply use the button with two green arrows up/down and you will see a list;
the most comprehensive options I ever saw; one will suit your requirements.

Sorry for my bad expression.

I meant, how could I devide the images in groups best. With tags, moving them in different folders, somehow mark them, etc.

That's the best question at all and more up to each ones philosophy of ordering things.

For chronological reasons I leave my images in folders where I have put them after downloading them from the camera.
In same cases I create different folders for different place where I had took them.
But mainly for particular subjects, places, towns, themes etc. I use the category's from which you can create your self as much as you want by right click on the category filter, > icon. You can also delete the original cats if they don't fit you.
The category's has the advantage that your order in the origin folders is not affected. By double click on an image you come to the folder.

May be that's the problem, I do not have any philosophy.

But mainly for particular subjects, places, towns, themes etc. I use the category's from which you can create your self as much as you want by right click on the category filter, > icon. You can also delete the original cats if they don't fit you.
The category's has the advantage that your order in the origin folders is not affected. By double click on an image you come to the folder.

Yes, using those options above all let one access the (grops of) images via different criterias. The disadvantage is that the images are accessible by that creterias only with XnView and not in another programm like Lightroom. Grouping the images by folder, so just grouping them in different folders would let them access the same in e.g. Lightroom and XnView.