No, if you're starting from today, you put the car's mileage (kilometers) that are on the car as of today. So if your car has 12,750 km on it when you put gas in, that's the first line.
Second line is how much gas went in to fill the tank.
Third line is how much money per liter.
If you're starting from today, the average is going to change every fillup, but get more stable after a while, when your average has more to go on.
I had old data to input from another program I used, so I went back and put in seven months worth of data into AccuFuel when I first got it. If you have the information, you can always add older dates too.
Hope that helps.
Edit: I realized that we're talking about two different entry methods. If you want to calculate km driven since last fill-up, enter 0 for your first line the first time. If you want to go by total km and have AccuFuel calculate the miles driven, do what I said above. Sorry for the confusion.