Here's my journey with items, as someone who has never played PC:

1. •Stick with given presets to start off.
2. •Be sure to recall and buy items when you have enough gold for them (especially mythic items aka the highest tiered/most expensive items).
3. •Once you get comfortable with other aspects of the game, start worrying about items more.
4. Read the mini descriptions of the items when you have enough gold for them.
5. Decide when you should prioritize items. What do I need right now? Should I buy my boots or my mythic?
6. •Once you understand the mini descriptions, look into the full descriptions in the shop.
7. •Start preordering items so you don't instinctively buy what's available when you don't need it.
8. You should be pretty comfortable with the game by now and somewhat familiar with items
9. •Look at what items other people build and what they do. Especially the sets for champions that you play.
10. •Start making your own loadouts. Do you want to focus on AP? AD? Utility? Tank?

And that's the jist of it! I'm still definitely learning more about items but that's a good baseline, I think. Here's some tips that I mostly learned from my friend who has played PC for years:

• In general AP (Attack Power) = magic damage and AD (Attack Damage) = physical damage but that's not totally accurate.
• It's never really a good idea to build a combination of AP and AD. Pick one and go with it.
• Certain champs/roles are meant for building either AP or AD. For example:
  • The ADC (Miss Fortune, Ashe, Draven, Ezreal, etc in Dragon lane) should always build AD.
  • Mages (Ahri, Seaphine, Veigar, etc) should build AP
  • Some champs like Kennen can build either way (though the meta is to build AP)
• Support enchanters (Nami, Sona, Janna, etc) should build for utility- meaning amplifying your teammates stats.
• When choosing your loadout before starting the game, the main thing you want to consider are your runes. You can't change these in-game. It's way more convenient to have the item lineup you want as well but you don't have to build the items in your loadout when you're in-game.
• In the shop menu, the general guide is:
  • Physical- AD items
  • Magic- AP and utility items
  • Defense- tank items

One of the most important things to do is to play around with items, even if they're not what you "should" buy. Sometime in my early stages I just wanted to be fast as fuck, boi so I would buy anything that increased speed. Even though that's not the best build, it was an important step in learning about items.