Have you tried looking up guides for the characters you want to play? It's the fastest way to learn how to generally build a character besides learning for yourself. (Why should you when the knowledge is at hand?) This however, will not teach you how to build situational.
You'll have to try to analyze for yourself why a X champion would build X item. Look at their kit, and look at the item, see if they have any synergy. For instance, why do some ADC's (like Corki, Ezreal, and whoever else) build Triforce? Because they have mixed damage, in their skills, AD and AP, making them hybrid damage dealers. Primarily these champions would focus on pure AD after Triforce, however, because ADC's are mostly all about Auto Attacks.
Last Whisper is good to go for when your opponents start buying armor.
With most melee champions, like Pantheon, Jarvan, Rammus, etc, are initiators, and you can't initiate fights effectively without any tanky items, you'll die instantly if all of their team focuses you right as you engage. Unless you're really far ahead, you should build tanky, you can build some damage early, say, a Brutalizer, but if you reach mid game and you're not very far ahead, you should start building tanky. Are their Ability Power damage dealers fed? Prioritize Magic Resist. Is any of their Attack Damage dealer's fed? Prioritize Armor. Sometimes you may see a mix of this, if you do, look at the ratio of AP champions they have compared to AD champions and make a decision based on that. Tanks would generally be neutral, unless they built some damage. (maybe from being fed)
Generally I think you should look up guides for any new champions you pick up, both for builds, or perhaps learning how to play them. This game is a lot about experience, knowledge, and mechanics.
If you want to learn general League of Legends mechanics, I suggest unswlolsoc youtube channel, look for Lecture 1 and continue from there: https://www.youtube.com/channel/UCeGtVfZD9LVttCJzjIJLY1g
It's has very informative guides on the basics of League, and even advanced mechanics which you could get into later once you feel comfortable with where you're at.