iOS

Since you don't want whomever gets your iPhone to have access to your personal data you'll need to wipe the phone clean of all of your personal data. Follow these instructions to clear the data off of your phone.

1. Tap the settings (gear icon) from the home screen (or whatever page it happens to be located on your iPhone).

2. Tap the "General" settings menu item.

3. Choose the "Reset" menu item.

4. Tap on the "Erase All Content and Settings" menu item.

But this won't completely erase your data from iPhone, and the data can be easily recovered by some data recovery tool.

So, I suggest you try Safewiper iPhone data eraser to wipe your iPhone clean without recovery before selling.

Source: http://www.safewiper.com/ios-data...
(continues)

As I said in my last mail, you can wipe your all content and settings by performing the "erase all content and settings", but this will not erase your data from iPhone permanently, may be easily recoverable.

Safewiper is a professional data wipe software which can help you erase everything from iPhone.

http://www.safewiper.com/ios-data-eraser.html »

Hello,

Can you please tell us what exactly you are going to do once your iPhone is reset?

So I can explain it in a better way.

Thank you,
Tandy

Based on my personal experience, I tried 3 different tools:
1. iTunes
Before using this method, you should make sure that you have synced your iPhone to the computer in recent times.
2. iCloud
This method requires your Apple ID and password.
3. Paid third-party software
such as a useful windows unlocker called joyoshare ipasscode unlocker, it also has mac verison.

1. "Erase All Content and Settings" on iPhone "Settings" to factory reset it;
2. Use a data eraser tool in the market to do this job for you.

Note: Sync and back up your important data first to iTunes or iCloud so that you can recover data with tool, like D-back, Joyoshare iPhone Data Recovery, Dr.fone, etc. after erasing data.

There are two ways to erase all of your data on iPhone.
1. Go to Settings > General > Reset > Reset all settings.
2. Using iTunes to restore the iPhone, which will remove all data on your iPhone.
In the end, if you want to recover your data one day, any professional iOS data recovery such as TunesKit iPhone Data Recovery is recommendable. No matter whether you have a backup, this software can directly bring your data back from the device.

Transfer or reset your iPhone by going to Settings > General. Try one of these things: Get ready to move your settings and content to a new iPhone: After selecting Get Started, adhere to the on-screen directions. Return to Settings > General > Transfer or Reset iPhone after finishing, then select Erase All Content and Settings. Get rid of all your info on your iPhone: Simply select Erase All Content and Settings.