Narration question for my writing friends...

In the narration of a story, should people be referred to by first or last name? In my latest story, right now, I have everything as "blah blah blah," FirstName said. But, there are some characters where we don't have first names, so they are "blah blah blah," LastName said. But that's just inconsistent, isn't it? So, should I make up first names for everyone? Or change everything to LastName said?