Login script login

Hi All, Having a grey brain moment here and need some advise on the logic of this, should be simple, login script. I am checking validity of customer number customer email customer password (md5 in mysql) So i have my form with relevant fields Now i am getting problems with either sql or how i am handling , and showing, and errors..... I think what i am asking is this If someone just hits the login button show error "All fields must be entered" If customer number dose not excist show relevant error If customer number ok but email not show error If customer number ok but email ok but password is not show error If all is ok set sessions, got this ok, and proceed. Any help with with this is very much appreciated. Kind Regards Dave C