We are going to do 3 ways to earn Geo, early game, mid game and late game.
Guide to Earn Geo Fast
In Greenpath you will find Moss Knights, they give you 40 Geo each but if you have Unbreakable Greed they drop 20-60% more Geo than normal. So go around and kill as many of these as you see, when you are done go to the main menu and go back in and you will be back with the amount you collected (You will be on the last bench you sat on and all the Moss Knights will Respawn).
Go to kings station and go all the way to the left until you see the steps to Ancient Basin , then loop back around but go on the top steps, and go to the top door and sit back on the stag bench (make sure to kill everything on the way around).
Go to Kings station, go all the way up till you hit the colosseum of fools, you have to do the first one then you will unlock the second, the second trial will give you 2000 geo every time you beat it, this is the best way by far but it will take some time and skill.