usually horizontally if another paper is to be hung over the top so the joints beneath are less visible.
some would argue that horizontal is better as joints are less visible through paint as you don't get the shadow lines caused by natural light coming through the window.... personally i disagree, i think that bad joints will show no matter which way they run. conversely good joints won't show regardless of orientation.
having said that, good practice dictates that one should always paper AWAY from sources of natural light, a rule that is worth observing.
if you are a relative novice then make life easier for yourself and hang vertically. take time and care to get your joints nicely butted up, no gap or overlaps.