I don't have access to my php.ini file since i have a shared hosting plan. When i try to add "php_value magic_quotes_gpc off" to my .htaccess file, it shows 500 internel server error message. I also tried to add "php_flag magic_quotes_gpc Off" to the .htaccess file.
Same error message again.
Please can anyone tell me how to solve this problem?

You will have to ask your host.

BTW, there seems a common misconception that because you are running on a shared host you do not have [access] to your own PHP.ini file. This is simply not the case. Any reasonable provider will provide you an individual PHP.ini file and some means to edit it. If that is not the case with your provider I suggest that he is not a reasonable provider and you should consider looking elsewhere for your service.

No, not all My host does not provide a php.ini as well, but I can do the necessary settings via htaccess as udaraaka, too (also my host is quite nice and even changes settings on the server directly if you ask them - you can't say that about many shared hosts).