You'll probably get more helpful answers if you take the time to write a complete sentence. Some people see this and think, "The OP doesn't even care enough to write out a complete sentence, so I'll only spend a few seconds answering the question."
Trading, items that you do not want or need that other players might want. Other have mentioned finding items people are selling cheaper than the average is and buying and re-selling. If on PC there are a few addons MM etc etc that help you know the average price of some of the sales your guild have sold.
Stealing: Going around stealing items from NPC's doing the steal two times kill the third (if you have Dark brotherhood DLC) also containers, desks, etc etc. Keep the green, blue and sometimes purple to sell in thieves den
Dungeons, doing dungeon runs and looting everything that drops and cashing it into a merchant after gives some good and quick gold. Goes the same for trials.
Clearing Inventory: kinda the same above anything that you picked up in the world and do not need go to a merchant and sell.