Runescape, the free massive MMORPG is a extremely fun game, but can be frustrating when you don't have any money. There are several methods of make money however, including, fishing and cooking, or mining, smelting and smithing.
First, fishing and cooking. These are the EASIEST SKILLS TO ADVANCE! And with a cooked lobster (acquired at level 40) you will get 250gp. First, you should net for shrimp and later sardines south of Lumbridge. Later, when your skills go up, acquire the necessary items (which are easy to find) such as fishing poles from the shop at Port Sarim. I went up to level 43 in this skill in only a few hours. While you collect fish, cook them, raising your level. The fish will sell at a general store for a decent price for the effort put into catching them.
Another method is Mining, smelting, and smithing. For noobs, it is best to start mining south of Lumbridge, collecting an even amount of copper and tin, go to Lumbridge and smelt the ingredients, and then travel south to Varrock with a hammer to craft the newly acquired bars into weapons and armor. At first, selling your product will not obtain a high income, but once you get to the point where you can make iron you will make some real money. If you continue this process, even though it is slower than fishing, you will make a LOT of money when you can make adamant items.
I hope you find this helpful!