tips on keeping things level and evenly aligned

hi , just wondering if there is any tips on keeping things level when building with walls and floors. say perhaps if building corridors , where sometimes they will turn straight down or up. or when adding a wall to a floor. as when you first bring a piece out it is horizontal , the the walls and such are vertical.

is there a way to attach 4 sides together(like a cube with 2 sides open) and resize or copy and paste all sides together? thanks