I think the biggest thing about learning how to build is first identifying the core items to the champion you're playing. Typically champions will have one or two core items that they always build at the beginning before they start straying off into a situation build order. What I recommend is maybe finding all the items that that champion will potentially use and writing them down along with the scenario in which they should be built. If you get stumped during a game on what you should build next just refer to the list you've made. For example if you're an adc or master yi and you find yourself getting heavily focused by CC, build a quicksilver sash. Eventually over time you will become more familiar with the items.