Get your mining to level 30 and mine coal. It will take a while at first but once your mining level is higher you can mine a tone of coal. Then go to Varrock and people buy coal for 100 – 150 RuneScape gp a piece. Or if your mining is 40 you can mine RuneScape gold ore and sell them for 200gp a piece.
Get steel bars and make kite shields, chain mail or platebody. Out of every 28 steel bars you will make about 8k+.
Mine 1k pure ess in a low populated world like 39, and then go to world 1 and sell it for 100gp per rune stone.
Get your mining and mithing to level 30. Start to smith coal and iron. Then when you have around 1000 of each with you, turn them all into bars. After that, go to an anvil and turn all the bars into steel or iron weapons/armor. Sell to the general store. (Its best to do this Falador, because you have the dwarven mines and an anvil + furnace).
Get your character's mining level up to RuneScape power level 38 by mining iron, etc. Then, go to the Barbarian Village and mine coal from the rocks, which you can sell for 100 to 300 GP per piece. You will also occasionally find a gem (from least valuable to most, sapphire, emerald, ruby, diamond) which can be sold for good RuneScape money prices.
Start by merchanting ess. You need at least 4k ess to start it, so mine those and sell for no lower than 40 each which will give you 160k. Once you’ve completed that, start buying in East Varrock Bank for maximum of 30 gp each.
Do the Knights Sword Quest then your smithing lvl should go up to 36 so you can make all the iron RuneScape items. Make armor and sell it for 500gp.
Get your mining lvl up to 20 or over and mine RuneScape silver. Get as much RuneScape silver as possible. Smelt the RuneScape silver ore into a silver bar. You can sell them in the general store for 30 RuneScape gp each but then they will buy them for 15 RuneScape gp.