Hi & welcome here.
I assume you've the form but, if not, here it is NIE DOWNLOAD
Here are instructions to fill it in (according to http://www.idealspain.com/pages/information/movingtospain.htm )
HOW TO FILL IN THE N.I.E. APPLICATION FORM
1 DATOS PERSONALES means PERSONAL DETAILS
1er Apellido: Surname
2º Apellido : (second Surname) Write three chiffons instead (---)
Nombre : Christian Name (make sure to use same as in passport)
Fecha de Nacimiento: Date of birth (use two digits for day and month)
Lugar de nacimiento: Place of birth
Sexo : Gender “H” for Male (Hombre) and “M” Female (Mujer) . X cross the one applicable
Estado Civil: Status. S for single C for married. V for widow and D for divorced
País de nacimiento: Country of Birth
País de nacionalidad: Current Nationality
Nombre del padre: Father´s name
Nombre de la madre: Mother´s name
Domicilio en España: Address in Spain (if you have one, otherwise leave blank) - Localidad: Town, CP: Postcode, Provincia: Province
Reasons for application:
Economic, Professional or Social (Tick professional for work reasons or social for house purchase)
3) DOMICILIO A EFECTOS DE NOTIFICACIONES
Sign the form at the bottom, under 'firma de solicitante'.
Complete the form and take it back to the National Police station, along with a photocopy. You also need to provide your original passport and a photocopy. That's it ! You will then be told to return in approx. 4 weeks time when your number will be ready to collect. You need this number to do anything fiscal in Spain such as open a bank account [this can also be done with your passport number].