Two lane main is all that's needed to function in Ranked, although I personally recommend you master all the lanes so that you can learn the entire game's mechanics on an at least operable level. (Let's phrase it like this. If you went into blind pick, hit the random button, and played whatever champion you happened to land on in that champ's normal role, do you have confidence in winning the game?)
The goal of the ADC is to be the team's gold sponge and general damage source. There are three main ADCs, the standard ADC or Hypercarry, the Burst ADC or Caster ADC, and the Utility ADC. Hyper carries want to focus on defensive play early to midgame until they get their items, in which case they become a late game DPS monster. Think Vayne, for example. Caster ADCs have bigger Early to Midgame power spikes, in trade for not having the raw damage potential that Auto Attacking Hypercarries have in the Late Game. They're more flexible like Lucian or straight up a whole different type of damage threat like Graves. Finally, Utility ADCs lack the raw punching power of the other two types of ADCs (in context, they still do plenty more damage over time than most other champions of other classes). What they lack in damage however, they make up with some other way to support their team. Ashe's slows and Stun are recognized by all League players, and an entire Teamcomp style exists based on Sivir's Ultimate. The primary reason the ADC is bot lane is to get their power spikes through gold, subverting the problem that is the Support stealing levels. Thus, you better make sure you know how to farm. ADCs are also the best class in Sieges, typically, and sometimes are the fastest for dealing with Neutral objectives, so if you learn to pick up this role, it's not rare that you will often be the centerpiece to any strategic Objective plays.
The goal of the Support is to protect the ADC until they're self sufficient, or complement them to dominate the lane. There's a certain saying; Supports carry the ADC early game so the ADC can carry the team late. If you're working with an especially proficient support, there's no end to the truth of this line. As the Support, it's a matter of balancing your pool of defensive Supports that defend the Hypercarries to offensive Supports that synergize with your Caster ADCs to shut down the enemy team to more flexible Supports that can continue complementing the rest of the team after the ADC offers their utility. Learn how to read the lane, like when it's best to push or freeze the minions, keeping track of cooldowns so you know when to engage, or pulling back when you're suspicious of danger.
Mid Laners are typically the secondary carry of the team, and look to complement their bot lane. For example, if your bot lane is Burst or Utility based, a raw damage DPS like Cass or Azir become solid choices, whereas Hypercarries would synergize better with Bursty Mid Laners. Once again, you can break the role into three generalized groups of DPS based, Burst based, and Utility Based. Burst Based champions want to shut down other squishies, DPS champs want to scale up, Utility needs to know how to complement the team well. Mid Laners tend to have much more different jobs, even between two champs in the same category, so their play styles tend to be a bit more champion specific. Generally though, Mid Laners need to either shut down certain targets (Tanks or Squishies, depending on the Mid Laner in question), or learn which of the three objective based targets they exceed at destroying (some Mid Laners clear lanes safely and effortlessly, some are still massive threats to towers, and others are great at either killing or setting up perimeters around Neutral monsters)
Top Lane and Jungle tend to round out the other three roles. For example, one of the two almost always want to be Tanky or have an Ability that simulates Tankiness given that the Mid Laner and the ADC are almost always damage based. Maybe your Mid Lane is a Bursty Assassin and you have a Sivir bot, you'd want to fill this out with a strong DPS champ like Ryze or such. Maybe all three primary laners are carry roles and you ought to pick a Utility Tank into the Jungle. So on and so forth. Unfortunately, this lane is even more champion specific than Mid. The only thing that really stands out as a consistent play style is that Top Laners need to know how to take care of themselves, and Junglers need to know how to leverage objectives.