Bottom line
Before you can go on any active duty, you have to be screened, your orders have to be cut in MROWS, and you cannot start travel or duty until you hold a signed copy of those orders. While you are on active duty you can use host base facilities like the AC, you accrue leave at 2.5 days a month once a tour hits 30 straight days, and you can get paid in pieces (an In Progress Payment) for short tours. Follow the admin steps in order so your pay, benefits, and leave all line up.
What it means for you
This section is the checklist behind every set of active-duty orders you execute. The big ones to remember: do not travel or start duty without signed orders in hand, watch your leave balance closely at entry-level schools so you do not check back in owing the government money, and know that short active-duty tours (30 days or less) get an In Progress Payment so you are not waiting until the end to see a dime.