New to PDF forms please help on one form field.

I am using adobe acrobat 8.3

i want to make a new form that will auto add a unique number to one field when the file is opened.

the name of the field is "Client ID" 

i have searched and searched and searched and yet i am at a loss i need some ones help, i am not understanding any thing i am reading i am doing every thing people ask of me that i read and yet nothing  is there any one out there that can help me or  make this one field form for me i can add the rest of the information to the form i just need this on the form

the form will be emailed and printed and emailed back or mailed back, the total pages is 10 pages long.

PLease help me some one.