First, thoroughly cleanse the iPhone as described here:
What to do before you sell or give away your iPhone, iPad, or iPod touch - Apple Support
Then follow the on-screen directions to set up your iPhone.
First, thoroughly cleanse the iPhone as described here:
What to do before you sell or give away your iPhone, iPad, or iPod touch - Apple Support
Then follow the on-screen directions to set up your iPhone.
Where are you trying to sign in? What happens when you try? Error messages(s)?
Apple ID is not working