Reporting Internship for the first time

Hi guys! i have an internship for the first time on D-10 visa. The contract stated that starting date is 16th march (yesterday) but i will probably send out the fax tomorrow (18th) because i just got the other documents from my company today. Is it okay? or does the starting date have to be like a week after? Also, i don’t have my ARC yet. do i just send the part in my passport where it states i have D-10?