Assuming you don't pick up the 4-year scholarship, get in touch with the NROTC Unit's recruiting officer and let them know that you want to join the College Program. As a CP, you'll still be a Mid, but you won't get any money. As nittany said, you can't really be involved if you're not a Mid.
Just get involved, put yourself out there, and get good grades. As a CP, you'll be eligible for a 3.5, 3, 2.5, 2-year scholarship, or advanced standing (stipend for junior and senior years). Chances are, if you're not a -bag and you're not an idiot, you'll pick up a scholarship (no guarantees though).