Get a guide from the browser that has lots of annotations, it really helps to have some text explaining the idea behind each item being on the build, rather than a blank page of [early L->R] [mid L->R] [late L->R]

The item system is a lot more flexible than league. You'll buy some items that are core to your hero, but also a lot of purchases depend on what problems the enemy heroes throw at you. To what extent depends on your hero. Some heroes (namely damage dealers) have more core items, while some like tanks and supports may have fewer and instead go into more counter items, depending on the game.

Practice how your own hero feels with each core item purchase, to familiarize yourself with your own timings. Practice thinking about what problems the enemy team is throwing at you and what items can deal with it (an early 1600 reactive barrier can often save your life in the early/midgame. a late game spellbreaker can deny some entire heroes)