How does one know how many years before year 0000(current years) say "300 BC" was?
There was no year 0. The structure of the current calendar goes directly from 1 BC to 1 AD (or AD 1, as you prefer). AD 1 translates as "the first year of our Lord"; the folks coming up with the calendar back then didn't have a proper concept of zero (zero as a distinct number didn't reach Europe until it was far too late for inclusion in the calendar).
The lack of a year 0 is why pedants can point out that the third millennium started in the year 2001, not 2000. Years 1-1000 were the first millennium, 1001-2000 the second, and 2001-3000 the third; if we'd actually had a year 0, this wouldn't have been a problem, as the millennia would go 1st: 0-999, 2nd: 1000-1999, 3rd: 2000-2999.