Your tension is off and that is why it is showing the dark thread on the top .
As for what way to press the seams it all depends on the pattern, colors , etc. It use to be to press to one side but since so much is now machine pieced and the stitches are tighter, you don't have to worry as much about the stress on the stitches. but with white you might want to press to the other side so the colors don't show thru.