In HOK, you get gold from minions even if you don't last-hit them — but you get about 30% more gold if you do last-hit. Over the course of a 15-minute game, that 30% difference adds up to roughly 1,500-2,000 gold. That's a full item. That's the difference between killing the enemy ADC and leaving them at 10% HP.
Practice last-hitting. Go into practice mode, pick your main hero, and just last-hit minions for 5 minutes. Do this once a day. Your CS (creep score) will improve dramatically within a week.
General CS benchmarks:
• ADC: aim for 8+ CS per minute
• Mid: aim for 7+ CS per minute
• Clash: aim for 6+ CS per minute
• Jungle: don't count CS, count camps cleared