Gold

The Gold is the only in-game currency, and you start with 475 gold to buy your first items. Then there are several ways to generate/get more gold in League of Legends:
 * Striking a killing blow against a minion or monster.
 * Each time an enemy champion is slain, gold is distributed to the Champion who strikes the killing blow, as well as any other champions who participated in the kill.
 * Destroying a Turret.
 * Certain champions have abilities that add gold under certain circumstances.
 * There are items, runes, and summoner masteries that generate gold in small quantities at five second intervals.
 * Champions generate gold slowly over time, and you will notice that your gold total is constantly increasing once the battle begins.
 * You can also sell items that you have already purchased for a fraction of their cost.
 * You get a +100g bonus from first blood

Ways to get Gold
Gold from minions: Gold can be earned by landing the killing blow on enemy team minions. The amount of gold earned is dependent on the game time. The longer a game of League of Legends goes on the more gold each minion is worth.

Gold from monsters: Gold can be earned by landing the killing blow on neutral monsters. No matter how long a game lasts, monsters of the same type always yield the same amount of gold. For instance, a wraith monster will always yield the same amount of gold regardless of when during the game you kill it. Certain kinds of monsters called epic monsters give gold globally, providing gold for the entire team regardless of which individual champion landed the killing blow.

Gold from Champion kills and assists: Gold can be earned by killing enemy champions. The gold gain associated with a champion kill is determined by how many kills the slain champion has recently accumulated (champions that have killed many enemies without dying are worth more gold). The last hitter always gets 100% of the value of the bounty. An Extra 70% of the value of the bounty is put into a gold pool. The pool is split equally among all champions who assist(damaged the slain champion within 10 seconds of his death). If there are no assists, the pool is lost.

Gold from destroying a turret: Whenever someone destroys a turret, every member of the team receives 100 Gold, no matter whether he participated or not.

Gold from Champion abilities: Some champions have abilities that add gold under certain specific circumstances. For instance, Ashe's ability Plentiful Bounty adds additional gold each time she scores a killing blow on a minion or champion, and Katarina's Voracity ability adds gold each time an enemy champion that she has recently damaged is slain.

Passive gold gain: The gold a champion earns without actively killing units is called passive gold gain. Every champion passively generates gold at a rate of 7 every 5 seconds. This gold gain does not begin until minions spawn at each base. Passive gold gain can also be the result of runes, summoner masteries, or gold generating items like a Philosopher’s Stone.

Selling items: Items that you find you no longer need can be sold back at the shop for 70% of their total cost.