On the N-400 form for naturalization, do I leave blanks or N/A on the sections that do not apply to me for ex... Spouse info

Which one of the two options is acceptable: leaving it blank or inserting N/A in the form. Also, when mailing the N-400 form with the fee in the form of a check or money order, what is the best method of mailing these important documents? Can it be done over usps express mail or priority? Thank you for your insight.