Trying for the first time to figure out mileage per state for doing ifta taxes. Like, I make a trip from Georgia to Texas, how would I figure out the miles? And I only bought fuel in Alabama and Texas, does that matter?

That is what the odometer is for. Otherwise, any Atlas will give you driving distance or just do a Mapquest between addresses or nearest cities on your drive.

You would have written down starting and ending mileage, it is your duty to keep records of mileage used. And to be able to produce those records to IRS if needed.

IFTA taxes are for long-haul truckers, who maintain detailed logs as to where they drive and their mileage.