Peter is correct, you do not apply for a nom until the yr you apply for an SA.
I would say look at the profile for the nom request, and start lining the recs up. MOCs vary some want educators, some will take anyone.
Additionally, the SAs will want their own recs from teachers, traditionally Math/Sci will be one of them. Before the school yr ends ask the teachers you want to write a rec for their email, and inform them that you will be contacting them over the summer.
You should also check with the gc or administration to find out when sealed transcripts will be ready for rising srs.
Noms are needed, but you need to also be academically qualified along with Medically for an SA apptmt.
Spend the spring and early fall taking the SAT/ACT because they superscore...no harm no foul
If you have a medical issue start working on the issue now, because nothing is worse than going through DodMerb as a candidate with an LOA, but not mq.
Noms open the door, but they are not a guarantee for an apptmt. Don't just worry about one aspect and forget the others.