The most popular ways to become rich

1.Fighting monsters
2.Crafting/Smithing
3.Mining/Fishing
4.Fighting other players
5.Trading
6.Scamming/Hacking
Fighting monsters – monsters drop a lot of good stuff, it’s not bad way to get money if you have high fighting skills.

Crafting/Smithing – making amulets, armor and weapons for money. You need lots of money to pay for gems, gold and ore. I don’t suggest this job for RuneScape newbs.

Mining/Fishing – you can get a lot of it but it is very boring and painful.

Fighting other players. You need a good warrior and a good team. Killing other players is fun but it’s risky.

Trading – buying armor, certs or what ever and selling it for a bigger price than you bought it. I think it's the best way to make money.

Scamming/Hacking - another way to get money, but it is illegal!