The first part you can do from any other computer on the same network as the Raspberry PI. Make sure the Raspberry PI is turned on and that you can connect to it as a web server. (Follow the guide about setting up a personal web server if you are unsure how to do this).
To create the WordPress database open up a browser Window and navigate to PHPMyAdmin on the Raspberry PI. The address will be something like http://192.168.1.xxx/PHPMyAdmin. (Replace xxx with the last three digits that correspond to where your Raspberry PI is located).