dont really understand how to use skill and how to trade and so on
btw im a magician

Press C to set your skills to alt, ctrl, or shift. Click and press that button when your charge builds up. Most skills charge up on attack/cast. It will tell you.
Use the trading forums to bid and auction off your gear to players, then use the in game marketplace to finalize those transactions.

is it true i can get fgs by selling items?

Only if another person buys the item, there is no merchant where you can sell item vs fg.

http://ladderslasher.d2jsp.org/index.php?g=4

Browse around in the menu to the left, you will find answers to 99% of your questions there.