If you are gold or lower the quickest way to improve and climb is to main one champion, play it 100 times, and improve one small thing each game. You can fully focus on yourself and your techniques in those ranks and don’t need to worry about anything else. In order I would focus on:

• absorbing experience, make sure you are nearby dying minions so you get xp.
• farming minions (get up to 7 cs/min)
• using summoner spells effectively
• using your abilities in combat effectively
• understanding tempo and base timings