If the field doesn't have to be a number, just set the
Data Type to text.
If the field has to be a number, in table design set the
Data Type to Number and the Format to 000 or whatever you
want. Entering the number 1 would display as 001.
You can't. Numeric data will always strip leading Zeros.
You may format the display to include Zeros, both in the textbox and in
reports but storage will not change
if you are storing Zip Codes, you should store it as text, not as numeric
data.