Responsive design – Most common resolution steps

I am developing a web app that needs to be available on desktop, tablet and mobile.

I know already about responsive design, media queries and all that stuff, but my question here is more about resolutions.

So, this has probably being asked several times already, but i'd like to have some up to date details about it.

If I have to design a responsive app, what steps do you think I should use for media queries?

I found this website that provides quite a lot of details about screen resolution in Europe (I am only interested in Europe):

But the resolutions displayed here are quite a lot.

Do you think that using the default 768px for tablets it's still correct?

And what do you think should be the smallest supported resolution? 320px or more?

Thanks for any feedback