Help with Item's

For adc's :

• what should I look at when deciding to go crit or on hit.
• what should you build when facing Ap malphites, Leblancs

For Tanks :

• when Is guardians horn worth and not worth it. I see it blocking a lot some games and not much others,
• Do you take tear/fimbul on any mana - using, tank
• If theres an adc popping off, like trist but the rest of the team is like, syndra, zac, lulu and nunu . Would you go plated caps or merc treads.

Any help is appreciated.

Feel free to post some common / noob build mistakes too, as I or someone looking at this post might be making them.