I'm in low ELO but I will share some knowledge I've acquired and what I think about it.
You should only focus in one champion IMO, and the reason behind this is because there are over 100 champions and you need to learn how to counter them this implies and by doing so, each champion has a counter but it would imply that you need to learn other champions and each have different stats and your ability to last hit will be affected, timings and such. Master one, and keep a few others that are similar to this champion so you don't have this issue, that said, you should try to play as them and try to figure out their weaknesses, not master them, just learn about their skills sets, damage ranges and apply this with your favorite champion. It will be overwhelming, try to stick to a few and have your favorite champion always ready.
As for roles, roles are meanliness if you aren't aware what they truly mean.
A toplaner can be a tank, a bruiser and sometimes an APC or an assassin, but they have a role in toplane to fill in the team, as for why they take TP, however in low ELO, TP is not useful if your team lacks communication. Would love to take TP, but if your team is tunnel vision most of the time and won't react on time, you are more likely to die and wasting TP, though there are some exceptions. There are champions that heavily depend on CS to get strong, those do need to take TP such Nasus and Jayce, but the reason why you want TP is not for just ganking or assisting for dragon, a toplaner primary job is splitpushing and you should splitpush only when your TP is up, and at the same time have your team go for objectives, but like I said, low ELO, you can splitpush and tell your team to take other objective and what is more likely to happen is you dying to 1V3-5 and your team did nothing.
TOP LANE
As for Toplaner you need certain skills: You need to master warding, controlling the wave flow, the knowledge of when to play passive and aggressive and how to play from behind.
Mastering warding sounds ridiculous, but I did said that each champion is different right? Well, certain champions can gank in a different way meaning they can avoid your wards, if you are going to push, you need to place wards deeper, know where to place your pink.
Here is a good video so you could understand, for this season.
https://www.youtube.com/watch?v=Fr2L3I5bWhM
Controlling the wave is a bit hard, you need to know how to mess with the minion wave, but this is important if your laner TP or is somewhere else, you need to push your wave hard. Most players go for soldiers and then for mages, you should to the opposite to this, mages deal the most damage meaning that if you take down the mages first, your minions will have more health to attack the turret. I always clear them first when I want to push hard. Sometimes you need to miss CS, and this happens to be the mages again, not killing mages will cause the wave to be pushed under your turret, this strategy is for forcing your opponent into your lane, you are preparing for a gank.
The knowledge on how to play aggressive and passive, is simple, but tiredsome at first. You need to check the minimap every 5 seconds. If you notice the jungler is in botlane, mid is busy you can play as aggressive as you can, but if you haven't seen them, do not push your lane, try to control it and wait for it. Warding will help you a lot for this too.
And finally playing from behind, there is a reason why tanks go toplane, if you died once. It's okay, play the same, but if you died two times you need to stop engaging, and you will not try to farm cause his person out trades you and can kill you, instead you just be nearby the minions and obtain XP, and try to use a skill to farm, but you know they can't get to you. Playing from behind means not dying anymore, just getting XP, but at the same time you didn't feed a champion so your team can deal with them later on and you still have your levels and aren't behind in the stats, eventually you will start to obtain gold and catch up.
JUNGLE
A lot of people mistake this role as someone who needs to win the lanes for everyone. This is one of the hardest roles to play because it involves tons of decision making, this is the leader of the group. As a jungler your role isn't to win the lane for everyone in your team, this is also important you don't need to balance the lanes (gank the losing lane cause the winning lanes are good), this is more like you gank the winning lanes even more to tilt the enemy and you secure objectives as well.
This requires a lot more studying than other roles IMO. You need to master warding too so you know where you should gank from, master a champion in the jungle that usually avoid those wards such Rek'Sai, Lee, Rengar, Evelynn, etc. You need to ask yourself a lot of questions. The most important question: who has an escape and who doesn't, those who doesn't, are your top priorities. Now, who is over extending? If your team is overextending who has an escape and who doesn't, those who burned their escapes are most likely to get ganked, so you can do a counter gank. Which turret can fall, which lanes to gank and which to forget. Some laners have no CC, without CC ganking said lanes are harder so avoid ganking them until they obtain a form of CC and you need to learn the priorities of dragons, turrets and who needs to be shutdown early, such Nasus, Jayce and if you kill them once, you can gank them again cause they are more likely not to except a gank.
There is just a ton more to learn from them, but you should watch some youtubers about this. RedKind, Nightblue3, Cowbell, Pants are Dragon, Foxdrop, and among others.
SUPPORT
This is a role I've been playing for 3 years, supports are actually second to junglers, as they can gank other lanes and they prepare and help to secure objectives. A Support and jungler have a similar role and as tanks their roles are similar and in the same sense, the support is there not only to support ADC's, it is important to know when to lean your lane and gank mid or help the jungler counter jungling.
As well you need to know when to leave your lane, master warding, when to gank, who to gank, set up baits.
As for filling in, this is harder, cause one fill won't help much specially if you got 4 squishies in your team against 4 tanks in the other, but if they are skillful they can win, but it is not that common.
As for me. I am learning every lane, and dealing against all sort of champions, though I mostly play Taliyah now days I play her in almost every role. This is something important, you need to find a way to win against your counters instead of learning a new champion. Why? Cause it improve your mechanics, by a lot. I hope this wall of text helps xD
in the game! So don't feel like you're going against yourself doing so. But once you have done enough exploring through all the champions and roles, nearly all high-elo players have a main role, and a large number also have a main champion in said role.
The reasoning for this is because if you're more efficient in one specific role than all the others, why would you not take advantage of that to climb? Use that win rate to your advantage! The same would go for a champion. Once you know exactly how to play/build/etc a champion, in a number of match up/scenarios, you have experience on your side.
Hope this clears things up at least a little bit!
in top lane and you have 
. Your jung/bot/mid start to converge to dragon and they tell you to teleport down to help them. The only enemies you can see on the map is their jungler, outside dragon pit to try and steal at last second, and their bot lane, starting to move up river to dragon to pressure your team. So, right now it looks like a 4v3 for you. That's good. At this point, you definitely should not teleport, irregardless of what your team is asking. Push that tower, right now it looks like you can get tower, dragon, and maybe a kill or two post dragon. Suddenly, you see the enemy top and mid lane come in from somewhere you did not have vision on, and suddenly it's a 5v4 for the enemy team.
Now you have two options. 1) Teleport in hopes to even the fight out and secure dragon, or 2) push top and get a tower.
If your team has taken substantial damage trying to take dragon already, i'd stay top and push unless you think you can 1v3 for certain, because you're going to have to if 3/4 of your teamates are half health.
If your team is relatively healthy, dragon is low and you know your jungler is going to commit to sticking around, and you have faith that a 5v5 could go well for you guys, teleport asap to be there.
It's just decision making in times as these that slowly sway the favor in your way.
