In order to hard reset a Sony Ericsson J300, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Sony Ericsson J300 device will be brought back to the default configuration of a brand new device.
1. Turn on the device.
2. Starting from the Home screen, go to Menu > Settings > General > Master Reset.
3. Phone will now reboot and restart. Done!