Arrows come in stacks of ten; click on a stack of arrows and you can add them to your Quiver. The higher your range level, the more arrows you can store in your quiver.
Note that if you trade your quiver to another player, that player only gets the quiver - they do not get any arrows that are in the quiver! If you lose your quiver, and get a new one, then all of your arrows will still be in your new quiver. Do no be scammed by someone claiming he is selling you a quiver full or arrows! Arrows in a quiver are not tradable!
To shoot an arrow, wield a bow and click on a monster and select "Attack". If the monster is not fighting another player in hand-to-hand combat, he will retaliate against you. You will continue to shoot arrows until the monster is dead or you run out of arrows.
Unlike the Combat Skill, you can attack an enemy someone else is fighting. If a player using Combat gets the kill, they get all of the experience. And if a Ranger gets the kill, the experience is divided between the Ranger and the Combater. This is called a "Team Kill".
You need 45 Smithing, one Iron ore, and two Coal ores to smith your own arrowheads. If you click the arrowheads in your inventory, you can "Make Arrows" if you also have a log and feathers.
There are reports of a "Mysterious Trade" smuggling in poisoned arrowheads from other games. If you can find him, and pay his price, you will be able to poison other players in the Badlands. Just carry the poisoned arrowheads in your inventory, and attack another player with your bow. If you successfully poison the player, you will lose one poisoned arrowhead.