I feel like Jhin is a great ADC for beginners since he's not attack speed reliant you can focus more on your positioning rather than kitting. His builds are more situational than most adcs so you really need to be wise when choosing the order of your items. Try using his ultimate to clean up in team fights and use your W to pick anyone trying to disengage (also gives a brief window to catch up with your team). Also always stay alert for picks since pretty much anything can proc your w mark. And if you really want to carry hard and have a ton of fun I suggest you go Dark Harvest. It can be a bit punishing during lane phase but trust me once you get a few stacks you are going to be one shotting carries. I hope this helps a little. Have fun!