You can use ..
=TEXT(NOW(),"dd/mm/yyyy hh:mm") (and you might want to swap the dd/mm part depending on your location & standard date format)
.. but then your cell will contain text, not a date/time. That means you wouldn't be able to, for example, subtract two such values to determine the difference between them.
If you want the cell to actually contain a date/time then you would leave =NOW() in the cell and custom format that cell (or column etc) as OGCV1 described, or perhaps as just dd/mm/yy hh:mm (again, you might want to swap the dd/mm part depending on your location & standard date format)