If you really want to be a better player
Choose a champion to main. Over 80% of your games should be played on this champion.
Which champion should it be? Most people will say "choose someone you really like." That's a solid way to choose your main.
If you don't have any preference, start by looking at what Challenger and Diamond 1 players are using as their mains. Those champs are already proven to get multiple people into high elo.