The register_globals directive can be enabled by creating a custom php.ini file containing the following line:
register_globals = On
Upload this file in the osCommerce parent and admin directories within your web hosting account.
Bear in mind that the directives in the custom php.ini file do not apply recursively to all sub-folders. Therefore, if there are other PHP scripts that require register_globals=On, you should copy the php.ini file to their directories as well.
If you are having problems enabling register_globals for osCommerce, contact your host for assistance. SiteGround provides the best osCommerce hosting and such requests are handled with ease.