Hello,
the easiest way of managing multiple resolutions is to use
Viewbox.
Create your UI using your rootGrid and when you think that is ready go to Document Outline or click right click on rootGrid > Group into > Viewbox. I usually work with 1366x768 or 1920x1080.
I dont really prefer Viewbox but is the easiest workaround, otherwise you ll have to use grids with stackpanels and play with Auto, Locked etc.
Read Guidelines for supporting multiple screen sizes
And test your resolutions Run Windows Store apps in the simulator