There are any ways to get money. All of the ways I am going to list down need certain traits that are very useful.
First of all is the widely used Merchanting skill. To put it short all you do is buy low and sell high. The advantage in this type of money-making is that you do not need to have any requirements. But even so it is best you have a moderate combat level.(This is because people do not consider you a noob when you a have a good combat) Also it would be more profitable if you have some money to buy some high level items.(Dragon dagger(p++) is a much sought after item.) It is also good if you are patient.
The second way of making money is a bit more dangerous and may need a few requirements. Dragon killing is a good way to make money even though it is highly dangerous and the beasts are usually avoided when possible.
The best way to get to them is to go to Gu'tanoth for which you need to do a quest named Watchtower. To get here you can use a Ring of dueling to Castle Wars or watchtower teleport to the watchtower near Yannile. This is for people who do not have a Dusty key(Taverly dungeon dragons)and for people who would like to fight blue dragons.It is best you range them if you do not have a very high combat level. Recommended weapons for range are Rune crossbow and mithbolts(a crossbow is preferred here because you can also wear a shield) The most important thing you have to carry is an Anti-dragon shield or better....a Dragonfire shield You also do need to carry food if you are ranging. DO NOT attack the Ogre shamans if you have not finished Watchtower.
There are some other ways of making money like selling Raw material(ores,food etc.) Some ores like Rune ores are very costly and are much sought after. Sometimes, just get a few combat runes and some unnoted high level food(sharks, swordfish and the like)and sell them on a highly populated PvP world. Hint: It is safer if you get some teleport runes so you can teleport away if attacked.
There is another way of making money but this is the most dangerous way and has a lot of requirements if you would like to be successful. Pking or People Killing is very dangerous but very satisfying if you do it right. For this it is best if you have very high combat stats(85+ in range and magic is highly recommended) DO NOT bring armor or equipment that you would regret losing. Also do not underestimate anyone for they could be players having high stats in everything but combat.These people have been known to kill high level players also. It is best to bring one space teleport methods(tiny Elf crystals, teleport tablets etc.)
All these methods are based on my experiences and sometimes maybe flawed. Do not consider them to be the best ways to make money. There are many other ways and it is best if you experiment and find out the ways in which you are most comfortable.