| Right-Click the cell then pick "Custom" from the Category for the cell format.
In there it should automatically pick the format of "00000000" but you can type it in if needed.
In formatting, 0 is a forced digit (inc. leading zeros) and # is an optional number, so wouldn't include leading zeros.
Eg:
01234 as 00000 = 01234
01234 as ##### = 1234 |