Do I use firstname or username

Simple question, but kind of confused…
I’m following some tutorials online to build a social network website.
That person is using firstname and username. I placed both in my phpMyAdmin / table sign up sheet.
On the sign up sheet, I just have firstname, but throughout the scripts, he’s using firstname & username.
Q. Should I just strictly have first name and change username to firstname?
Please Help!
Thanks,
Scott