If the decimal is to the left of the numbers, you go to the right until the first value that is greater than zero and start counting then. If the decimal is to the right (23455.0), start counting at the beginning of the number and then round off once you reach how ever many sig figs you need. So if you needed 3 sig figs, round it off after the the first three numbers (23500.0). Sorry, significant figures are confusing and hard to explain.