Do not play Lee Sin. I repeat do not play Lee Sin. Lee Sin is very difficult to play. I am a high plat Rengar main and I play maybe a mid/high gold Lee sin at my very best. He is incredibly challenging and requires you to have tons of mechanics in order to play.
But, here is what you do if you EVENTUALLY want to be a good Lee Sin player. Find a champion like Xin Zhao, Mundo, or Volibear (very easy to play, do tons of damage, have good ganks, get quite tanky) and just spam them. Literally every game you can play that champ you pick him and play him. Play many, many games with the champion you picked. (it might sound like a lot but 150-200 games on a single champion will make you improve a lot)
Focus on your jungling. How could YOU have made that gank go better? How could YOU have farmed better while still be a presence for your team? How could YOU have won the game for your team. Can you predict where the enemy jungler might be? Were you placing down your trinket wards around objectives and in the jungle. How is YOUR pathing?
If you can't answer these questions, find a high elo jungle main stream and pay attention to everything they do and try to figure out why they jungle the way they do. Ask them questions.
Notice a common trend with the questions though? They all revolve around focusing on things that YOU can effect. Sure your team will lose you a lot of games and you will lose your team a lot of games, but at the same time you will win your team a lot of games and they will carry you many games. What matters is the 10-20% of games that are super close where every decision you make will directly effect whether you see a victory or defeat screen. Whatever happens though, remember there is ALWAYS something you could have done better. Be humble in victory and gracious in defeat, because every loss is an opportunity to learn.
So now think about these 10-20% of games that are really close. Would you rather be playing Lee Sin where you not only have to focus on playing a hard champion but you also have to make complex game decisions or would you rather be playing a straightforward champion that allows you to focus on winning the game? Definitely the latter. If you play an easy champion and focus on trying to get better as a jungler it will make it much easier to play Lee Sin in the future. Why? Because you won't have to think as much about actually jungling and can then focus on honing your Lee Sin mechanics.
Long story short, hold off on the Lee Sin. He will always be there later to play. Trust me in that he will be much more fun when you are a better jungler and can pull fun plays because you already know how to jungle well. :)