You have a small priority list while playing, and there's no real right or wrong way to play as long as you can follow them.
1.Protect your towers
4. Deal the killing blow on minions
5.Kill enemy champions
6.Control neutral buffs, dragon, and Baron
In the meta-game
1. call missing persons from the lane, anyone not in your vision is a threat to your other lanes
2. don't push any lanes while a team is missing and ward as many choke points and buffs as you can within the jungle.
3. time buff respawns
4.any advantage, no matter how seemingly small is an investment towards the rest of the game
5.Just because you're legendary, doesn't mean you're unkillable.
Follow those steps in summoner's rift, and you'll tend to win, try to think whether or not an exchange is worth it. You know sometimes ideally, you should save everyone and ace the entire enemy team without being scathed, but that's also unrealistic, you have to often calculate the risk/reward, and see who will come out on top in an exchange. It's unlikely you'll 1v5 and ace a team, and it's bad to try. It's arguable, but you can trade a dragon for top tower, and it would be considered a larger gain for your team as a map advantage.