Some Tips
The goal of this game is to kill the enemy Nexus! Killing the opponent is not the main key for that.
If u fuck up before objectives, objectives are pretty much done, there is no need to send somebody there to hero ball it. 90% of the time that person will die and u will snowball your opponent even harder. Flipping drakes or nash like that is really only a good idea if your jungler has the opportunity to 50/50 it(or with Ults like Ezreal, Lux)
Fighting or chasing while your opponent can push a sideline for free is in most cases worthless. U might get 400g from a kill but meanwhile there is a whole turret gone or a couple of plates and your opponent gets a load of gold for his team and himself.
Lane priority is key if u want to prepare or start objectives or fight in the river f.e. if ur midlaner is pushed under his turret and u start to fight the enemy jungler at the crab then the opponent midlaner will always have the chance to move first. Same goes for every other lane.
I could keep on but those things might be the most important.