How should I connect an outlet to two white and three black wires?
That's  because  your wiring is in conduit, which is actually the deluxe way to do things.
I disagree with the advice you are getting regarding &quot;hook the supply wires to LINE and the other wires to LOAD&quot;.   You should not use LOAD at all, ever, unless you are doing a very specific thing, you know exactly what you are doing, and you comply with Code in doing it.  That &quot;thing&quot; is extending GFCI protection to other outlets.  You must label those outlets &quot;GFCI Protected&quot;, which obviously requires that you know which ones they are lol.
Protecting random downlines-you-know-not-where seems like a &quot;can't hurt, might help&quot; sort of thing... but actually, it can go badly wrong several ways, and you don't have the experience to resolve those problems.  It is likely to turn into a fiasco... Just avoid LOAD altogether as a general rule.
Those wires don't appear to be color-coded.  My guess is you are looking at
2 whites = neutral (supply and onward)
2 blacks = always-hot (supply and onward)
Solo black = switched-hot
If there's a switch somewhere that appears to do nothing, that may switch  this wire.  (provision for Christmas lights?)  Perhaps they wanted to give you the option of a &quot;split receptacle&quot; (one socket powered one not).  That won't happen with a GFCI obviously, but you could choose whether to attach the GFCI to the unswitched or the switched wires.
So you'll be putting 1 or 2 wires on each LINE terminal... read the GFCI instructions for how to do that, and torque the screws HARD (preferably to spec with a torque screwdriver).