Continuously practice your mechanics
First, you definitely need to practice how to hit and dodge champion spells. Commonly, you can do that by playing a lot of ARAM, or you can visit websites like LoL Dodge Game, where you can practice and perfect your dodging skills.
Find a role you like, then practice 3-4 champions of that role. And look for guides on how to improve in that role. To then focus on how to improve at a general level