First realize that there are two basics to learn in league, champion mechanics and game knowledge. Dying a lot might not have anything to do with your mechanics but rather your lack of game knowledge.

So if I were you I would stick to one champion (preferably one you actually like) and go into bot games/training mode and just practice the mechanics of said champion. Google guides and youtube videos and study the champion mechanics. Every champion has his/her learning curve and just practicing combo-ing abilities is a good start. Some champion combo examples are lux E->auto->q->quto, or lux flash->q. A good channel to look at is Lol Dobby who showcases high level players on different champions and breaks down their mechanics.

Once you are more comfortable with the mechanics of the champion you can hop into real games and start focusing more on the game knowledge, which you can study but really comes from playing the game more. The idea is that since you are not focusing too much on mechanics, you will start noticing patterns that occur when you die. The more patterns you realize the less you are likely to fall for that death again. For example as a new ADC I died a lot early game but then I realized that I kept dying because the enemy ADC/Support get level 2 first while i was still level 1 which gave them a massive edge over me. So I learned to either attempt to get level 2 first, or to back off and play safe till I get my own level 2 spike. With that I then realized more patterns that there are some ADC/Support combos that are better at getting level 2 first than others so that is something I keep in mind now when I pick my ADC.

There are a lot of patterns and things that can give your enemy an advantage such as Creep Score, tracking cooldown or managing wave states, but again these come with time and playing more and I wouldn’t stress too much on it.

TLDR: Practice mechanics in bot games. Afterwards play vs humans and start watching for patterns to better your game knowledge.