There are three prominent ways to make money on Runescape. The so-called big three include mining, fishing, and woodcutting.
Mining
To get started, you will need a pickaxe. At the beginning, you will only be able to mine tin and copper. After a while, you will be able mine coal and make more money. Since copper and tin have very little value, you can either combine them into bronze or just drop the ore. It is not recommended to keep it in the bank.
A miner should sell ore for as a high of a price as is possible. If someone offers you a lower price, it's best to hold out. You will want to advertise in crowded areas, such as Varrock Square. Be on the lookout for smithers. They are often willing to pay more than the going price for ore. Remember that keeping and maintaining friendships with buyers is essential. If someone is buying higher than average priced ore, establish a relationship and add the buyer to your list. Always keep your promises. If you say that you will sell a product, sell it.
Fishing
The first thing you need to consider when preparing to fish is the equipment. You need something to catch the fish with and possibly some kind of lure. You also need to consider what kind of fish you'd like to catch. Once you have that determined, you can choose where to fish and exactly what kind of lure to use.
Fishing takes time and practice to be able to make a profit. You can begin with shrimp from Al Kharid and move on to trout from the Barabarian Village and Shilo Village. Later you will be able to catch lobsters from Catherby or the Fishing Guild. Once you pass level 80, you will be able to catch sharks and sell them for anywhere between 700 and 1,000 gp each.
Woodcutting
Woodcutting is perhaps the easiest of the Runescape skills. You begin by wielding your axe. Choose the tree you wish to cut down, and chop at it until it falls. Once you reach level 60, you will have cut around 4,000 willows. You can sell them for around 30 gp each and get around 120k for them. Once you reach level 60, you can cut yews and sell them for 300-375 gp each. Another tip is to buy them for 250 gp and sell them later for around 300 gp. There are numerous ways to make fairly easy money woodcutting. It's just a matter of experience.
By gaining skills in these three areas, you can decide which one suits you best and work at it until perfection.