1.What is Gold used for in this game? because u cant use it in the auction house?
2.is the only way to get Diamonds by using real money and selling stuff on AH or is there other ways to get diamonds?
3. im a Wizard so if there is anything I should know, any tips, would be great.

potions medkits
auction and dailys
don't know the official forums has builds

Is there any professions i can do that make crystals?

leadership makes some

With leadership You can do max cap everyday with just a few clicks after you level it up. You can also invest some of that AD to buy better assets to make that even quicker. Using a few heroes You can make 50k+ AD every day and not even play or log into the game.