how can you earn money in runescape?

You should of at least said your level well anyways

3-20:kill cows and take there cow hide for 100 gp each

21-40:mine rune essence

41-60:fish lobsters(f2p), kill druids (if you're member)

61-80 (if you're member): hunt poler kybitts

81-138: collect snape grass, runecrafting guild, killing druids, and kill green dragons <<<<<< 81-138 are member's only

Sell things you don't need like old armor and stuff at the grand exchange. One warning though is that it doesn't always get bought right away. Keep in mind that it is real people who are buying it so keep the price kinda low. You can also sell VERY large quantities of useless items like tiaras. Chances are, somebody who is doing the same thing will buy them

Do quest, fight monsters and get drop, do slayer, fishing, Woodcutting, Mining, and doing other skills, or you can just go on google and use those cheap sites and trade in real life cash for some cash in rune scape, but i don't think you would go that far for rune scape cash.