In order to hard reset a Sony Ericsson J120, 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 J120 device will be brought back to the default configuration of a brand new device.


1. First, install the PC Suite.
2. Open Tools > Phone repair.
3. Follow up with instalation wizard. 
4. Voila, done. Your phone should be good to go.