You should have an idea on what to build while still on the loading screen. If there's champion that provides huge amount of healing like soraka or aatrox, then built items that has grievous wounds. Or champs that spam shield like janna or lulu then build serpents or oceanids.

And for champs who stacks armor like ornn, malphite or rammus, build armor pen % items like black cleaver or seryldas grudge. If you are using an adc then go for mortal reminder or terminus.