The easiest way is to found to make money quick is by selling rune essence. First, you must complete the rune mysteries quest, which is extremely easy. Then, go to the magic shop in Varrock which is just south of the east bank. Right click on Aurbury and click teleport. You will end up in this really cool ice place. Go to a big white swirly thing and click mine rune essence. Mine until your inventory is full, then store it in the east bank north of the magic shop. There is always someone buying rune ess in that bank. Gather up a whole bunch of ess and sell for no less than 20 gp each.