There are many methods to earn or farm on Runescape. Most important is to just enjoy the game, and take pleasure in the challenges you choose to take on. One of the most well-known methods include:
Flipping using the Grand Exchange - always one of the best options in any mmorpg which offers an international marketplace/exchange game. You must have Gold to begin with and some experience in order to succeed.
Enchanting Sapphire Rings
Making (smelting Iron bars, cooking Trouts and possibly a several other lucrative items)
The Gathering (cutting Oak Trees, lumberjacking generally is my top alternative)
Tanning Dragon hides
There is more information and details in the guide here.