I think that my iPhone has been hacked, what should I do?

Why do you think that?

To keep your Apple ID account secure, see the following Apple articles:

Manage and use your Apple ID

Manage and use your Apple ID – Apple Support (UK)

Recognize and avoid phishing messages, phony support calls, and other scams

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Phony "tech support" / "ransomware" popups and web pages

Phony "tech support" / "ransomware" popup… - Apple Community

Identify and filter junk mail in iCloud

Identify and filter junk mail in iCloud - Apple Support

If you think your Apple ID has been compromised

If you think your Apple ID has been compromised - Apple Support

Identify legitimate emails from the App Store or iTunes Store ->

Identify legitimate emails from the App Store or iTunes Store – Apple Support (UK)

Set up iCloud Keychain

Set up iCloud Keychain - Apple Support

Create a strong password for your Apple ID, It is best if your password consists of eight or more characters and include upper and lowercase letters, and at least one number. You can also add extra characters and punctuation marks to make your password even stronger. 

Never provide your password, passcode, security questions, verification codes, recovery key or any other account security details to anyone else. Apple will never ask you for this information.

See also the following Apple articles:

About the 'Trust This Computer' alert on your iPhone, iPad, or iPod touch

About the 'Trust This Computer' alert on your iPhone, iPad, or iPod touch - Apple Support

Authorize or deauthorize your computer for iTunes Store purchases

Authorize or deauthorize your computer for iTunes Store purchases - Apple Support 

Get a verification code and sign in with two-factor authentication

Get a verification code and sign in with two-factor authentication - Apple Support

Check your Apple ID device list to see where you're signed in

Check your Apple ID device list to see where you're signed in - Apple Support

It most likely was not hacked.

Most of the time when someone thinks their phone has been hacked it has other explanations

An iPhone is not at all easy to "hack" if you have the latest security updates it is very unlikely that it has been hacked.

If you want to wipe everything off the phone and start fresh you can:

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

If you want to speak with apple support:

Contact Apple for support and service - Apple Support

Good luck!