Well, if you are trying to just delete personal data off the phone, you need to check that you don't have a google account, or other account, synced to the phone. If you do, that data will keep being put back on the phone. So you would have to unsync the account while you do the delete, and leave it unsynced, or delete the data from the synced account as well, before you reconnect them.
If you are trying to completely wipe your data (all data you had on it) from a phone, then a "Hard Reset" would be your best option. This removes all you have put on the phone, and then resets it to the way it was before you started using it.
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the phone.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...
1. Make sure that the phone is actually powered off.
2. Remove your SD card, if there is one in the device.
3. Press the "Power" and the "Volume Up" buttons, together, until you see the start-up logo on the screen.
4. Once you can see the logo, release the buttons and allow the device to enter the "System Recovery Mode".
5. Next use "Volume Down" button to get to the "wipe data/factory reset" option When in recovery mode the "Volume Up" and "Volume Down" buttons are how to can navigate and highlight what option you need.
6. Once you have selected the "wipe data/factory reset", press the "Power" button to select that option.
7. On the following screen, repeatedly press the "Volume Down" button until the "Yes - delete all user data" option is what is selected.
8. Then press the "Power" button to hard reset the phone.
9. If you took your SD card out in step 2, when the device is up and running again, reinstert the SD card.
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