A php.ini file changes the default (or server assigned) directives that are given to PHP, like the memory limit. To create one, just open up Notepad or something similar and save it as php.ini. Upload that to the folder where your website’s files are.
I don’t know what you had in your INI file, but I can give you what Magento sets up in the .htaccess file (if your host has PHP running as a CGI binary, you’ll use an INI file, if you host has PHP running as an Apache module you can flag PHP directives in your .htaccess file).
You can learn more about what you can put in your INI file from here:
Here’s a complete, default PHP.INI file: