League of Legends: Last-Hitting

Why should I last hit?

Last hitting is one of the most important aspects of Summoner's Rift. It is the primary source of income, especially during the laning phase. Even if you round to 15g per minion, and you kill a total of 200 minions, that's 3000g, which is enough for a Bloodthirster or 100 short of a Zhonya's Hourglass. That's one full final-tier item worth of gold with the generous estimate of 15g each minion.
 
The only two roles that don't necessarily get too many last hits are Junglers and Supports in the current trend of the game. Junglers spend most of their early game in the jungle, which doesn't necessarily fall under last hitting. They do, however, last hit if they end up in a lane to defend it. Supports rarely ever last hit because they are usually paired up with someone in the bottom lane, who they relinquish their farm to. They typically only last hit if their partner isn't around so the gold isn't wasted.

How do I last hit?

In simple terms, you get the killing blow on an enemy minion. This is easiest with a ranged ability or character because it's safer. In most situations, you want to wait until the minion's health bar is as low as possible before you kill it so it lives longer in order to avoid pushing the lane toward the enemy tower. Keep in mind the timing of your attacks and abilities. If you are attacking frequently, your attack delay may make you miss a last hit.

Every last hit counts. Be aware of which minions are being attacked so you don't miss out. If three minions are at a sliver of health and you can only attack one, you usually want to snag the one that is being targeted by your own minions so you can maximize the number of last hits you can get. Also be careful when using AoE spells. They may weaken the minions to a point where you can't land the last hits, especially when your tower is also shooting at them.

To last hit well in a real game scenario requires good timing, positioning and awareness. You never want to endanger yourself when you go to last hit. Dying is the last thing you want to do. Make sure you're aware of where your enemies are, what champions they are, and how much of a threat you're in when you go for the last hit.

Last hitting is important, but it's not everything

It's great to last hit, but don't forget about the match. Going for last hits can often put you in danger, especially if you're away from your tower. Over extending for last hits is almost guaranteed to get you ganked. If your team is grouping up, you should probably group up with them. If your team is backing up to defend the base, you should help. Don't forget League of Legends is a team game, and winning a 4v5 is very difficult. Winning a 1v5 is even harder.