Finding your position/role is the first thing that a player needs achieve, but how do you know that's the right position/role for you?
By "role" or "position" i mean Top lane, Mid lane, support, "ADC" "marksman" or Jungler. Finding what you are good at and mastering that role is one of the keys to having a better chance of winning games, but lots of players just play what they want to play and never take the time mastering the role they are best at. This causes them to make mistakes that impact the end result of the game, not mastering your role or not knowing your role will keep you at low level skills.
How to find out what your best role is, is by playing all of them and seeing the one you are the best at, having a feel for all 5 position will give you an idea of the ones you like and are good at. Then you need to pick the role you are good at and stick with it, not just playing the role for the most "fun" you will get out of it, but for the role you are the best at. Because even if you play the role you have the most "fun" in does not mean you will have "fun" each game, because we all know losing a game is not fun and often frustrating.
If you don't know your best role or are just getting started then play 5 to 10 game in each role to find the role that suit you. Also playing a champion that goes well in the role, example support Thresh, Biltzcrank, Leona, Alistar...Top lane Riven Darius, Gnar, Tryndamere...Mid lane Brand, Viktor, Zed, Lux...ADC/marksman Lucien, Jinx, Corki, Ashe...Jungler, Lee sin, Zac, Vi, Rek sai. There are lots of different champions to play in each role more than i could put in the description above, as long as you don't cross pick you will get a good idea if you're good at the role. by "cross pick" i mean pick a support champion and go top with that champion.
Once you've found the position you're good at you should at least have a backup in case you don't get it. Example if you are playing rank and someone takes the position that you wanted you will need to play a different position. This way you will be ready if it happens to you, most of the time you can try to convince them that you main a role and are really good at it and they might give let you play it. Always try to play the role you are the best at, by doing so you will be able to master your role and will have a better chance of winning games.
Note: if you are under level 30 on your account then we recommend that you don't not try Jungler because you need good Runes and Masteries to survive and thrive off the jungle.