There are several ways to make money in Ragnarok Online, some easier than others. I'll go down the list and hopefully you'll find something that works for you.

  1. Farming Poisona
    Poisona is one of the most effective means of farming zeny on RO2, simply because the card it drops is the single most highest selling normal green card in the game. Now, that being said, the drop rate of this card is ridiculous, and you may spend hours without farming a single one, but once you do that single card can fetch an easy 230z at auction. Not to mention he also drops the green "of Resignation" set, which can sell for a decent price at auction, or be broken down into Oridecon/Elunium/Mysterious Crystal.
  2. Farming Looters
    Another means of farming that requires fighting a monster, the scratch thief "Looters" drop the ever popular Scratch Thief Card, which can easily sell for a cool 50-60z at auction. What makes this route more appealing than Poisona is 1) there are plenty of them around, as opposed to just 1, and 2) they have must less HP than Poisona and are therefore much easier to kill.
  3. Farming Materials
    This is probably the most common means of farming for zeny in the game, farming for crafting materials. You can easily return to the Mjolnir area where the Porings or Flips are, kill oodles and oodles of them, and pick up mats quickly and effeciently. I myself can usually walk away with a stack of 99 Firmament Crystals after only about 45 minutes of farming, and a stack of Firms can easily sell for 100z or more. Not to mention the other materials, which sell anywhere from 50-100z per stack respectively.
  4. Random Hard Dungeons
    Last we come to RHDs, which at anytime can reward you with shiny equips/accessories/etc from monster drops, and the ever popular Old Blue Box, which you receive from dropping the last boss in a hard dungeon, each of which holds a random assortment of items, ranging from potions to rare materials. The rare materials (Cracked Emperium Piece, Fragment of the Unknown, etc) sell for decent prices at auction, ranging from 80-190z.
I hope you found my guide informative, or at least somewhat helpful in gaining zeny the legit way in Ragnarok Online 2. I myself have earned up to 30,000z after a few days of constant farming, and you can too!