In order to change to the Cleric class, you first off had to choose the Acolyte class for your very first class change back in Siwa Island. When you reach level 15 you should be able to get a class change quest into a Priest(Which you have already completed and done), and when leveling further and leveled to level 30 you get another class change. There should be an auto-quest to go and talk to a class master. You can go to any class master in the 3 major cities. These are the approximate co-ordinates for the three major cities. Jale(X166 Y158) , Kaslow(X 179 Y 334) and Ilya(X 349 Y219).
Once you have found the class master you will have 2 class choices. Cleric and Sage. Make sure you pick Cleric as you wish to become a cleric.. Upon picking the Cleric class path accept, address and complete it accordingly and then hand it back in to the class master in any of the three cities mentioned.
Classes changes are at level 5, level 15, level 30, level 65 and level 85.
All other information that might be able to get answered can be found in the Class Guide/Information For Priest/Cleric/Sage