There is usually a fee to apply for a residence hall room, and there is almost always a fee to reserve a room. Otherwise reserve a room at your top choices. So you'll just have to decide that for yourself.
Speed is usually the key.
Residence halls fill up fast at many schools, so find out what it is like at your schools. At this time, the only major activities remaining are enrolling and attending Orientation. The way schools enroll their new students and provide orientation are so varied, that there is very little universal information I can provide. Whatever you do, do not underestimate the importance of orientation. There wouldn't be an orientation if you didn't need it.