If you are lucky enough to get an ANG slot for pilot training, they will send you away to OCS (officer commissioning school), and then to pilot training. If you graduate from UPT, with a recommendation to fly the type of plane at your unit, you should be assured a flying slot. If, on the other hand, you didn't manage to graduate UPT, then you'd usually be offered the ability to just resign your commission. I don't think that any unit would actually guarantee you a flying slot, as things have a habit of changing from time to time. Good luck with your search.
JJ