Well, being a one trick pony as far as a lot of Diamond streamers say, is not really that bad. A lot of player find there niche and just stick with that role and secondary role always. That is of course unless you are not having fun doing Top or Mid? You can learn these other roles but may find that you do not like them near as much or that you are not as good at them as you are at Top or Mid.
Ultimately learning these roles I would suggest watching streamers which I will recommend some of my favorites that you can learn a bit from.
ADC does not leave too much room for situational builds as Top, Jungle, Mid and Support do because you team depends on you to farm and have a damage heavy build and carry. That being said you deal with quite a bit of harassment while farming from a support, adc and sometimes jungle. Also you are usually the one that will be needed to snag a quick Dragon or fend for it. It is a fun role, dealing loads of damage and just really being needed but a lot of people cannot get the hang of certain mechanics such as Kiting making it a very hard role for people.
I find Wild Turtle and Imaqtpie to be some of my favorite streamers when it comes to AD, they play it well and sort of teach you the role.
Support can be difficult for people, I honestly have not found a support steam that I like and found super helpful (If anyone knows one please suggest it) Support to some people is very boring, all about warding , healing and saving the ADC. In a lot of cases , that is what it is. I find the best supports should have huge amounts of map awareness, ward deep into jungles, watch over objectives and basically provide as much utility to your team as you can. A role that a lot of people underestimate but truly is very important and a lot of people still do wrong.
Finally Jungle, I love jungle but I love it for the sense that I feel like I have control of my team. It makes me feel like the leader. I say that because you choose the plays to make (ganking wise and objectives) and you really have a lot of roam to ward up on Baron, Dragon and maybe lanes that are getting ganked quite a bit. Also with all the jungle changes this patch, the jungle is just so much fun. The buffs you get from everything along with the different machetes to match each play style make it a lot of fun to jungle. Once again this role can be done wrong quite a bit because I feel with this role , you are pretty responsible for keeping watch over everything.
I could go in depth about it but I find that Trick2G does the best job of explaining jungle and teaching. If you are interested in jungle then I feel he teaches it the best.
Just some things I have picked up and been learning. I am no Diamond Player by far but still learning and helping anyway I can! :)
Good luck in your journey!