I was able to go on the game but when i changed the resolution, I wasn't able to go on anymore. Everytime i clicked play it always says "Full Screen Error". Does anyone have a solution or a method in which i can change my settings and resolution without entering the game.
Thanks.

In the Shaiya directory (usually C:\Aeriagames\Shaiya) there is a file called config.exe
If you have file extensions turned off it will be the file called config that has the Shaiya icon.

Run the config file and modify the resolution and/or full screen mode.