I'm going to give you more in depth advice but sadly i can only give it from the perspective of the Support/Jungler. Starting this season season i was placed in gold 4, I've played 180 ranked matches with 140+ as support. with roughly 20 being jungler and the other 20 matches being split between the remaining roles.
I'm now Plat 2 I tell you this because it shows that even the most looked down role for being able to carry games can still have a positive win rate and have a large effect on the game.
Most of these games are actually solo queue though i certainly do recommend finding a good friend that is of similar skill level that doesn't play your roles is a boon to your winrate - sadly you may only want to play with positive people as negative/ragey friends can make the team play worse and even surrender a game that can be won.
Talking of surrender, never ever surrender a rank game I've won a 3 v 5 with the enemy team not knowing our situation and they surrendered and who knows the enemy team may get internet issues, may mess up a baron, may get wiped in a late team fight next to their base - the game isn't over until your nexus is in shards.
I think it's a lot more beneficial for you to decide on 3 roles that you wish to main and then practicing every different aspect of those roles whether it be CSing to ganking. Also to avoid roles that your poor in is another way of saving your ass from a terrible game.
Additionally you should go for selfish picks I know Janna is extremely powerful at the moment with every spell being a disengage however she relies on her team rather heavily and if your team aren't up to scratch then you no matter what you do you will probably end up with defeat. To counter this I really have to recommend Leona and Blitz these two supports can catch people out of position (this is commonplace even in plat) And make a hard to win 5 vs 5 into a rather easy 4 vs 5.
A good piece of advise is to find what the meta is - who is strong and who is not, and then out of the strong ones find the ones you prefer to play most - remember to also look at ban percentages lets face it if you want to play Elise you have to deal with an 80% ban rate which means it's unfeasible to even try to learn her.
If you're picking Jungler it's best to go for an early ganking champion something like Udyr/Xin/Shyvana rather than one that relies heavily on hitting 6 first e.g Amumu/warwick because you can guarantee that at least one of your lanes will lose and not know how to play defensively we've all been there.
Now for some advice for supporting, Always rush the sightstone and once you get the sightstone switch to sweeper. A really good way of remembering to do this is in item sets usually you only need to put the first part of your build in as late game depends on what occurs during the match.
Some people tell you to buy vision wards - you should only do so if you are WINNING, if you are losing your lane/cannot protect the pink wards do not even bother putting them down. It just creates an objective that you want to protect and if you aren't able to protect it your team technically lose 130 gold and worse case scenario if you do try to defend it and die you set your team back even further. The only good reason to put it down while your losing is to use it as bait when your jungler is nearby this is especially effective when their support is melee.
My next advise is pretty bad because everyone knows it but some people just really don't heed it. Don't die, death is a huge setback to your team, I've seen so many cocky ADC's run about with 200 hp in lane against a poke set up bot lane and die because they thought they could juke every skill shot it's better to recall and miss 10 CS than it is to die and miss 9 CS.
The second last advise I have is form it may sound ridiculous but form carries from game to game, if you had a horrendous game that has made you angry and frustrated I recommend not to play another ranked until you have cooled off but the opposite holds true, if you've had a good game you usually bring that positivity into the next game.
Sorry for the long read, and basic advise but only other thing i can tell you is practice, it takes a while but when you've played enough games you can eventually gauge your strength vs the enemies and should always be able to make decisions according to this.