Making logo in header smaller and fixing header

I uploaded my logo, but it looks like this on desktop: https://i.imgur.com/iAEEaAu.png
and like this on mobile: https://i.imgur.com/aDmgzjZ.png

The other problem is, why is there empty space above the header, as shown here? https://i.imgur.com/atdHamF.png

Thanks in advance,
Daniel.