Do I come to the post office to do a passport or do it online first?

Yes, you have to go to the post office to do your passport.

You have to be present to have the application filled out, so I'm thinking in person. You'll need an appointment.

There's a form inside the post office. You can check online for instructions for applications and renewals. If you want to do it in person, definitely check online to see what all you need to bring.