A good first step is to try and get a Merchant´s Guild, to do that train all your merchants in one city and refuse any other guild but the Merchant Guild there.
If you can identify a monopoly resource, train your merchants up on those, once they´ve got a couple of finance points you can either improve their skill further by attempting some takeovers or you can send them to the real money makers - the cloth at Constantinople or the sugar, spice and cotton near Antioch. There´s also a gold mine near Ragusa, the only one immediately accessible, if I´not mistaken, so try and occupy that, too.