Experience Through Practicing Skills
Using a skill successfully causes you to gain experience in that skill. If you cook food successfully without burning it, you gain Cooking experience. If you cut wood, you gain experience in Woodcutting. The same holds true for your combat skills: The more you fight, the more your combat skills increase. You can increase some skills through practice only when you undertake them in specific circumstances. For instance, the Slayer skill increases only when you are slaying monsters at the request of a Slayer Master.
Assist System Experience
You can gain experience by participating in the Assist System. This system pairs more experienced players with less skilled players so that the less skilled person can develop his abilities. For example, a player with a low Cooking skill is apt to burn food when trying to cook it. If a skilled cook assists, the player is more likely to cook the food successfully and thus to gain experience from cooking. The more skilled player also benefits from being awarded experience for assisting.
Quest Experience
You can gain experience through completing quests. These are groups of tasks connected by a narrative -- rescuing someone from monsters or finding a particular herb for an individual, for example. The quest doesn't have to be related to the skill you are trying to master. Some quests reward you with experience when you complete them. You may also be rewarded with access to facilities that make it easier to practice a skill, thus gaining experience.
Random Events
You can gain experience by dealing successfully with random events, unexpected occurrences that transport you to a new area where you must complete a puzzle or challenge before you can get back to the main adventure. Some random events award you a lamp as a gift, which you can rub for experience. The Surprise Exam random event can give you access to the Book of Knowledge; read this, and you can increase one skill of your choice.
Minigames, Distractions and Diversions
Distractions and diversions are regularly occurring events that you can participate in to gain experience. You can participate in a minigame at any time. Examples include the "Tears of Guthrix," a distraction in which you collect the tears of the God Guthrix in exchange for an experience boost to your weakest skill.