I'm assuming you're talking about college program...? If so, you are required to pick up advanced standing before they will even commission you. So until you get AS, no you cannot sign on. As to how early you can pick up AS, I'm not sure. But I don't think you can apply until the end of your sophomore year, and you're required to have it to continue on.
Again, though, there's no point to commit before you have to, unless you're looking for a "guaranteed" (nothing's guaranteed) commission. In that case, try to get the scholarship ASAP.
Hope this helps.