First Fuel Up, New car
 

I bought a Focus today and I filled up the tank to the top right after I left the car shop.

How do I add my first fuel up? I tried setting the "Kilometers since last fuel-up" setting as null (0), but it said to enter a number. Should I input the total car kilometers (83000 approximately)?

What should I input there?

It sounds like you're using tripometer tracking instead of odometer tracking. Fuelly relies on full tanks, so your best bet is to reset your tripometer now (at the point you filled your tank), and then add your first entry to fuelly next time you fill to the top.

There's no way to know when the tank was last full because you didn't have control of the vehicle. If you were using odometer tracking I'd say go ahead and enter the details of this fuel-up and you'd start getting numbers with your next full fuel-up.