You have a few options.

If you want to treat the value as text:
#1. Preformat the cell as Text (format|cells|Number tab|Text)
(then do the data entry)
#2. Prefix your entry with an apostrophe: '0012345

If you want to treat the value as a number:
#1. Give the cell a custom format like:
Format|cells|number tab|Custom category|000000
(to show 6 digits--including leading digits if required)

debbie wrote:

I am creating an Excel Document that has certain values that must have
leading zeros. Does anyone know how to allow leading zeros in Excel?

--

Dave Peterson