You are likely aware of this already: although the 3 year AROTC scholarship and related financial benefits are for the sophomore through senior years, you are expected to fully participate in the battalion's activities from the beginning of your freshman year. This includes attending the MS1 classes and labs as well as all morning pt sessions.
It would be in your interest to show up on campus in good physical condition ready to do well on your APFT and to exert yourself toward getting high grades in your classes. If you make a favorable impression, there is the possibility that your scholarship could be upgraded from 3 to 3.5 or even 4 years. This would be at the discretion of the cadre, assuming it has allocable financial resources.
A surprisingly high percentage of MS1s with 4 year scholarships do fail their initial APFT or even the first few tests. This failure does not result in the forfeit of the tuition portion of the scholarship but it does delay it. Their monthly stipends, however, begin only when they have contracted.
Congratulations on your scholarship and best wishes for your success at SDSU.