How to securely erase an iPhone 5

See this from Apple...

What to do before selling or giving away your iPhone, iPad, or iPod touch

https://support.apple.com/en-us/HT201351

To add...you may get more money for the phone if it is carrier unlocked. See this for more details...

https://www.att.com/deviceunlock/#/

I did followed the tutorial as it tells, but i don't know whether it didn't work. 

Not sure what you mean? Did you follow this procedure?

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1. If you paired an Apple Watch with your iPhone, unpair your Apple Watch.
2. Back up your iOS device.
3. Tap Settings > iCloud. Scroll down and tap Sign Out. In iOS 7 or earlier, tap Delete Account.
4. Tap Sign Out again, then tap Delete from My iPhone and enter your password. 
5. Go back to Settings and tap General > Reset > Erase All Content and Settings. If you turned on Find My iPhone, you might need to enter your Apple ID and password.
6. If asked for your device passcode or Restrictions passcode, enter it. Then tap Erase [device].
7. Contact your carrier for help transferring service to a new owner. If you aren't using a SIM card with your device, you can contact them to get help transferring service to the new owner.

Does the phone say.

Hello!

Hola!  

Slide to set up>

then it has been factory reset an has no information on it.

I was told by my freind that this  doesn't work on jailbroken iPhones. That's why the phone keeps flashing when i hit the erase button. After two days researching, finally got the problem solved with the following tutorial:{Inappropriate content removed}

Thanks all for the kindly suggestions.

If you had included that information, I would have told you you have to update with iTunes, effectively removing the jailbreak, then factory reset.