plzz hlp me understand it

reading this should introduce you to the game and its mechanisms: http://ladderslasher.d2jsp.org/index.php?a=1

additionally, you can find guides here e.g. about how to log in: Ladder Slasher Guide Collection

Basically playing the game does not cost fg, but buying items ( which helps a lot to improve your playing ) effects your fg.
But you can of course also earn fg by selling items or gaining guild points when you attempt to master quest.