Is this the postcard size thing you fill out and send back? You must say yes on that postcard (NO OBLIGATION by saying yes, forfeit by saying no or not responding).
Later, you should get more paperwork or phone calls to ask what your kid is going to do.
My sons NROTC "recruiter", who handles the paperwork at the MIT unit, said he would hold the decision for us when the time came and that we could simply phone him on I-day if son went USNA. He implied that a letter to his office would be appropriate, explaining that USNA was first choice and that son was committed to attending NROTC if for some reason USNA fell through last minute.
Congrats and Good Luck!