Depends on exactly what you want to do with the number.
To use as text with no calculations (credit card numbers - model numbers),
on a cell by cell basis, you could prefix the entry with an apostrophe (
'01234 ), which is *not* visible in the cell, but can be seen in the formula
bar,
OR, for large ranges,
*Pre*format the column (range) as 'Text'.
To use the numbers in calculations,
Custom format:
0000000000
Which will display leading 0's for less then 10 digit numbers.