Play every role. You'll find which ones you like the most. Pick 3 of those roles and get yourself at least 3 champions per role that you are reasonably proficient with.
Play every champion at least once. You don't want to enter into a ranked game and have to ask your team "what does Kha'zix do?" because that negatively impacts the team morale.
Buy wards. Buy at least one on each back. Learn to buy pink wards and pink ward your own jungle to prevent invades from happening.
Learn what freezing a lane is, how to freeze a lane and when to do it.
Learn how to CS properly. Even if you prefer to support, learn to CS.
Watch your own minions in laning phase. The enemy will try to last hit them. Make them pay an hp tax for every last hit they make. This will influence future trades in your favor.]
Don't be afraid to sacrifice yourself for an objective (towers, dragons, inhibitors especially) but don't die for no reason. Don't go roaming into the river alone to check for danger if it was pinged to you multiple times.
Call your MIAs. Give your bot lane a fighting chance against that roaming Katarina.
Know how to mute someone.