The best way is to pick up items other players dont want and sell them at the grand exchange, located between the lumbridge bank ( the big one with the side door and stairs for underground vault, near the iron ore mines) and barbarian village and the brass key locked place. I do apolise if your new to the runescape world and have no idea wat im saying but this is the best way to explain it. The grand exchange has a concrete path leading to it with jesters and and it has a massive group of people there typing words like selling full rune, or buying bronze scimiters ... Etc.
There you can sell and buy stuff using premium prices, buyers can find anything they want there by using a high bidding price, sellers can earn a lot of money, dont EVER sell stuff at the general stores, ( the stores with pots of flours as symbols) they buy your items cheap and sell them expensive. At the grand exchange it has set prices for every item in the runescape world, determined by how much people are willing to offer, it changes every day and like the stocks people sit there and wait , so you should wait for a good price and then sell. You have two slots to sell ( unless if your a member) and you just place the item that u want to sell there and wait, its either instant cash or waiting. It stays there until another buyer wants it and when you log on the next day theres cash instead. The good thing is that it runs for all the worlds so if your in world 101 and a person wants to buy it in world 13, your item gets transfered there and youll get the cash transfered to you. It good because you earn a lot, a pot is woth roughly 2gp at the general store but at the grand exchange its worth roughly 23gp, and you can set the price higher or lower according to how you want it, but if you set it too high no ones gonna buy it.

Another way is to buy a brass key and go into the trapdoor located between the grand exchange and falador. ( falador is a city that has mines and quests) there is where all the lvl 50s and above train, they dont pick up the stuff that they get after a battle and its worth thousandths of gps. There you fight hill giants and spiders and zombies and hobgoblins. It is pretty dangerous there because hill giants are lvl 23 but youll manage in a crowded world because a lot of people train there and sell the bigbones that the hill giants drop at the grand exchange for roughly 500gp and it will be instant buy, because they are wanted everywhere by rich, high level runescapers.
Good luck :)
Any more questions or problems, im happy to help. Varock (mind my spelling) is the main city to train, lumbridge ( where you find the castle and where you start of ) is a gathering city where you gather stuff, falador, is a mining city where u go there for rocks and ores and smelting them. Its also a quest place but the quest have low rewards so dont bother. Port sarim, is a fishing city where you can catch the boat to karumigar and get a banana picking job. If you ever get lost telaport back to lumbridge, it is in the middle of all the cities and everyone there is happy to help because most are beginners starting there.