You get a lot of very bad players who can be very toxic. These are people you 100% ignore and get on with your own game.
Bot lane main checking in - as adc, the two main things you need to know for lane is how to farm and how to trade. Farming is simple, but can be quite difficult. Creep gets low, hit it. Gain gold! Problem is, if you don't know how much damage you do, you can sliver creeps quite often. My biggest tip for last hitting is to watch what your own creeps are attacking - if you have 5 ranged creeps that've piled up and are going ham on some poor creep, you need to watch their particles so that you can time your attack with when the creeps get it low, and before they kill it.
For the supp who tries to farm: you can ask him nicely to please stop
You can see if he has relic shield and just doesn't understand how it works
And last option: just play assuming your supp will try to take the farm. If you see him going for a creep he can't last hit, be ready to follow up. As the adc, you'll pack more damage too, so after getting pickaxe or bf you can last hit earlier than he can, and keep your farm that way.
The hardest part of playing adc, by far, is teamfighting. I have 700 games of League and I still don't think I ever do max dps in teamfights. It's a tricky business of positioning and playing around cooldowns.
As a supp, it depends on what your champ is. You need to understand what your champ's role in a comp is to play it well. You can't play Janna like a pick champ, you can't play Blitzcrank as a disengage support. You need to understand your role and matchup to know what you should be doing - I think that's part of what makes support tricky. I suck at Soraka because I play pick, tank and disengage supports. I just can't sit in backline and heal people during teamfights.
Jungling: first noob tip is to learn how to efficiently clear. If your jungler uses a lot of mana, you can start Gromp -> Blue -> Wolves -> Red -> Krugs; if you have a good AD jungler, then starting Krugs -> Red -> Wolves -> Gromp/Blue -> Blue/Gromp is a good first clear.
You'll find at first that you'll only be able to do gromp and blue before having to back to base to finish your clear later. But once you get good at it, you'll be able to do a full clear on almost all junglers the first time around! Clearing is all about killing the camp as fast as you can, while taking as little damage as you can. How you do this depends on your jungler and you should really just look up guides per champ to learn their "combos" and tips.
Something unique about the jungler is that you can afford to look at your minimap 24/7 without missing much on your own screen since you're afk farming camps. You have three things to think about as a jungler:
1. Farming up
2. Snowballing lanes
3. Setting the other jungler behind.
When to farm, when to counterjungle, when to gank are hard decisions to make. IMO, your default mode should be farming, and while you're doing that, look for gankable lanes. The prime example of a gankable lane is:
1. Pushed up to your tower - this means they're far away from the safety of their own turret. It's very rarely worth ganking a lane that has someone stuck under their own turret, unless you can pull off a tower dive.
2. Is an immobile champ - Viktor mid keeps pushing up? Punish him. Ashe adc keeps shoving the wave? Punish her. LeBlanc mid is shoved up to the tower? LOL GOOD LUCK CATCHING HER.
3. Has summoners down. A lane without flash is so much easier to gank as it limits their escape potential. Keep in mind, if you gank a lane and they burn their flash to get away from you, then that's a successful gank! After that, flash is down for 5 minutes, which leaves them super vulnerable!
4. Raging in all chat. Tilt them even more by continually ganking.
5. Has healthy allies. You can't gank bot if your adc is on 5hp and you can't solo kill their carry. You baiting your adc to come fight will likely just get you both killed!