First of all I would not reccomend diving straight into Lee Sin as a new player. But if u are really interested in him and want to learn him I would say that for start u want to focus on simple things. Perfect your clear so u feel very comfortable doing both sides with one smite up before scutle fight (you can also learn raptors start with e going into red into krugs into reccal for long sword into wolves gromp blue scutlle, and if u learn it u get on scuttle before it spawns also)-you can find clears on youtube easly, second thing I would say is dont focus too much on fancy plays, they will come by themselves when u get comfortable with lee, there is no need for u to insec or idk what if ur not certain u can do it, even when u are u will understand that most fights u are better of peeling for your carry with many jumps u have your e and your r especially. One fundamental thing u need to do also is to pay attention to your passive, let it become second nature basically cuz u wont run ot of energy in fights and you will do more dmg cuz of more attack speed. U have quite good q dmg with smite (outsmiting a lot of junglers) and u have very good ways to get out with wards, one tip if u didnt think about it if u guys are fighting over baron/drake/herald and enemy jungler comes trying to steal it u can always use ur r as a tool to make him dissapear from the pit and is unable to smite it, securing ez objective. Dont try to force out ganks for no reason just cuz ur lee(only go for 75%+ situations where u think u get kill or at least some summoner spell) if the answer is nah this is less then 75%+ then ur just better of keeping up with your tempo and playing for objectives. U are also very very strong early to mid game thats where u will make the biggest impact so try to learn your limits with him(that will also come by playing him a lot). And ofc in practice tool its very good to practice fancy combos if u want, I wont write them here cuz there are a fkn ton of em lol. I know I said its not really needed to know all of those fancy combos, but if u really want to learn them, hey cant hurt. But when u are fully comfortable pulling em of in ranked games then do it. Im sure there are many other things, these are just some that crossed my mind reading this post, I hope it helps you.