Ray's suggestion will cause Access to select the entire field automatically when a textbox is entered, but remember two things:
If someone TABS from field to field instead of clicking, Access automatically will choose the entire field anyway.
Whether you tab to the field or click to it, if the entire field is selected (all black), you don't have to delete the zero first before typing your value. Just start typing. Because the field is selected, typing will automatically erase what's there first and then start entering your keystrokes. No need to delete first.
Michigan Dept. of Environmental Quality, Air Quality Division