Sorry if this is an incredibly basic question for these categories. Basically, I don't understand these types of problems. I'm sure it's something really simple I'm missing.

Let's say there's an open swimming pool with width $w$, length $l$, depth $d$, and density $\rho$ (equal to water's density).

So basically, I'm quite sure the formula I should be using for this is $P = \rho gd$. This turns into $\frac{F}{A}=\rho gd$

How would I find the force of the water exerted onto the sides ($w$ and $l$)? In a problem like this, what would the area $A$ represent? If I wanted to find the pressure on a $w$ side, would I use $w\times h$? I try this, but it doesn't work. I get $F = \rho gwh^2$. But this answer is double the actual answer. It seems like I'd need to integrate (that's what I tried first), but it didn't work either.

There has to be some really basic concept I don't understand.