Step 1: Making Money for Noobs (Newbies)
To make money with chickens go to a chicken farm (there is one in Lumbridge if you follow the northern path) and kill the chickens for there feathers. When you have around 1,000 or so go to the GE (Grand Exchange) in Varrock and sell them for the market price (usually around 6gp each).
MAKING MONEY WITH COWS
First to make money with cows, go to a cow pasture (there is one just east of Lumbridge castle and one behind the Lumbridge mill. The Lumbridge mill one is better because there is a bank deposit box there). When you get there take the cowhides that they drop when you kill them and when you have 500 go to the GE and sell them for the market price (usually around 114gp).
MAKING MONEY WITH BONES
To make money with bones go to a place that has creatures that you can kill quickly (For example: just east of Lumbridge there are low level goblins). Then kill as many as you can and pick up the bones that you are given when they die. Bank them and when you have collected 500 of them go and sell them on the GE for the market price (bones:76gp--monkey bones:193gp--burnt bones:192gp--big bones:339gp). (To get big bones you need to kill dragons)
HOW TO GET MONKEY BONES
To get monkey bones you will have to go to Port Sarim with about 200gp and talk to a captain or customs officer to get a ride (that is what the money is for). Take a ship to Karamja and go into the jungle. When in the jungle you will see monkeys kill them and they will give you monkey bones. There is no bank on Karamja unless you are a member so you have to go back and forth between Port Sarim and Karamja.
MAKING MONEY WITH MINING
To make money by mining first mine copper ore (you can mine copper ore south of Lumbridge castle) until your mining level is 15. Once it is level 15 then go just and south-east of Varrock and there is a mine. You will find 3 iron rocks (you can prospect them to find out what they are), mine from those rocks and bank them north of the mine until you have 500 and sell them at the GE (iron ore sell from between 99-101gp).
Step 2: Making Money for Medium Level Players
Level 30 fishing
Level 25 cooking (optional)
To make money fishing you should fish salmon at a river. To catch salmon you will need to go to a river with the amount of fishing bait as you want to catch salmon/fish. When you have caught a full inventory bank it and go back to the spot and keep repeating the process until you are out of fishing bait. When you run out go sell it on the GE for the market price (73gp raw and 60gp cooked)
MAKING MONEY BY WOODCUTTING
Level 30-60 woodcutting
an axe better than bronze
To make money with woodcutting you will have to have a pretty good axe and find a willow or yew tree depending on your level (yew: level 60 woodcutting--willow: level 30 woodcutting). When you have found your tree cut the tree(s) down until you have 500 of the logs. Then go to the GE and sell them for the market price. Willow logs: level 30 woodcutting and sell for 18gp--Yew logs: level 60 woodcutting and sell for 473gp.
Step 3: Making Money for High Level Players
To make money with dragons you will have to go in the Wilderness and head north until you find dragons. When you have found them make sure that you can kill them and that you have an anti-dragon shield on. Once you have done that then kill them and pick up everything that they give you (they will give you big bones for sure and green d'hides and some other random but good things). Kill them until you either have almost no health left or you have a full inventory. When one of these things happens then go sell you wares on the GE (Grand Exchange) for the market price (you will get 339gp for big bones and I don't know how much for a green d'hide).
MAKING MONEY BY MINING
level 30-70 mining
To make money by mining you have to get to a mine that has at least coal (it would help if you could go in the mining guild). When you are at your destination mine coal until you have 500 and sell it on the GE for the market price (usually around 177gp). You can also mine mithril ore, adamant ore and gold ore when you have a higher mining level for more money. Adamant ore: level 70 mining and sells for 1,014gp each--Gold ore: level 50 mining and sells for 501gp each--Mithril ore: level 55 mining and sells for 264gp each--coal ore: level 30 mining and sells for 177gp each--clay: level 1and sells for 99-101gp each.