In the Guard, you interview with a unit, they decide to sponsor you. You are in. You go to AMS (academy of mil science), then SUPT. When done with SUPT and training for your particular aircraft, you go fly with that squadron that sponsored you.
In the Reserves, you interview with a unit, they sponsor you. Your packet is then sent to Dobins,GA where the Reserve board meets twice a year (march/sept) and they decide if you will go to SUPT. If sponsored, you are 90% in. You compete with others sponsored around the country for a certain amount of slots. This March board 58 of 65 were selected. After being selected, you'll go to OTS, then SUPT, then you will go to fly with your unit that sponsored you. It is possible to send your packet to the board unsponsored but your chances are not as good, and if selected you have to still interview and find a unit that will take you. Do a search for any other questions you may have, anything you could possibly want to know about the Guard/Res has been asked in this forum. If not feel free.