First what you have to do is build your team. Don't just play right away. Before you do, go to auction search and buy a Rare Bronze player. You can find them because the text on their card is in white. Buy it for 300 coins and then put him on auction. Make the starting price 400 coins so you know you have a profit. Put the Buy It Now price to 900 coins. For example, I bought a player with high shooting skills for 200 coins. I put the Buy It Now price to 2000 and the starting price at 1000 coins. I sold it for 2000 coins so a HUGE profit off a 200 coin player. I have quickly earned thousands of coins using this method but there are more methods.
Be sure to check out http://www.fifaencyclopedia.com/fifa-12-ultimate-team-money-making.php as they give more methods to coin making