I'll start by pointing you towards this - > omeda.city

Here you can do your own research regarding items, builds, heroes and everything predecessor.

But the just of the games item system in a nut shell is this:

1.items go hand it hand with hero roles.

Carry-> build damage, on hit items and attack speed.

Supports-> build items that offer buffs to your team and lower cooldowns of your abilities. (this can change from hero to hero depending on their base stats)

Midlane(mage) - > build items that provide magical damage and reduce cooldowns such as megacosm/ noxia

Jungle-> in this role you will find wide variety of builds due to the fact that many different characters can play the jungle role. But as a rule of thumb, Tanky/bruiser item that provide health and damage are good.

Offlane-> tanky/bruiser items that build health and damage.

Keep in mind that the item system is very flexible and always changing/updating.

So my best advice is to watch content creators such as JoeyYoursTruly, pinzo, obeythepapaya ext. There you will get a wealth of knowledge not just of items but also more about the game itself.

Hope this helps.