[PHP Filter on a Form]

I'm a very novice user with html and php.  Website and php form is in place working correctly before i got here.  We receive many blank forms or forms with only one field filled out from users that must sit there and click submit.  Is there a way to force a pop up when not enough has been filled out.  Or if easier block the form from being sent.  Thanks in advance.  P.S.  A filter.txt file is already set up that has a list of bad words in it.