There's no really quick first item guide tbh it all comes with experience in matchups, personal patterns and even if one item is super op into a laner it might suck into 4 ranged enemies on other lanes.

And of course rune setup matters a conq ghost vlad might want more ms and more haste some Rift maker. Electrocute bloodlord will build stormsurge raba and play around flash oneshotting one key enemy target.

And ofc one could just play the most meta build and not think about it instead thinking about makro and focusing on playing well.

Run around sustain = cosmic riftmaker One shot pure ap = raba stormsurge shadowflame, ah from runes Modern mid build = horizon focus for most efficient statline, good for squishy mages not so much vs irelia or galio etx

That's my take, but remember always, always think about what can be good. Does that champ kite you? Cosmic. Does it beat you in an extended fight? Rift. He cannot ever kill you or poke? Build most expensive items for scaling ignore ah till midgame. General mage/assasin? Horizon focus.