The rules require you to have a clear line of sight to your target (in this case, the point of origin of the spell):
A Clear Path To The Target: To target something, you must have a clear path to it, so it can’t be behind total cover. (PHB, p. 204, or here in the Basic Rules)
There are also rules that (usually) require a thing to have a clear path to the point of origin for the thing to be in the area of effect:
A spell’s effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn’t included in the spell’s area. To block one of these imaginary lines, an obstruction must provide total cover, as explained in chapter 9. (PHB p. 204, or here in the Basic Rules)
But you do not need a "clear path" from you to a creature for it to be in the area of effect of one of your spells.
Note: Since Fireball states that "The fire spreads around corners" (PHB, p. 242), note that fireball would often affect creatures "crouching behind a short wall" even if the creatures were on the opposite side of the wall from the point of origin, and the point of origin was at ground level. But for other spells which do not go around corners (such as Shatter), this remains a viable and valuable tactic.