I still dont understand items

I have like 30hs in the game and I still have no clue about what items I need to buy, I always go for the recomended items and most of the time my teamates think Im trolling because X item doesnt work against X champion. There are like +100 items how can I know every single one of them and what is best against X champion or X item. In all of my good games I end up doing low to no damage and I cant tank anything because I never know what to buy.

Edit: Im reading every comment, thank you all for all the good tips and information! I love this sub