First fill up on brand new car?
 

Hi, Purchased brand new car, and had half tank of fuel, so topped it up, after it had done about 60 miles.

Should i add the full capacity of the tank and the zero odometer reading OR wait till first full fill up,(2nd tank full)? That way will have a odometer reading that will be accepted.

Basically, how do you start recording on a brand new car in fuelly?

You don't need to worry about a zero odometer reading or a full tank capacity. Just fill your tank full to the top and record the odometer at that point. And then repeat. So you start recording the first time you put in fuel.