How do you get a hotel room if you are under 18?

No. Not unless you stay with somebody who is 18 or older. Some hotels will allow it, depending on their policy. Quite often, teens rent a suite of rooms to party on prom or graduation night. Some hotels will allow it if an adult will register the room in their name and assume responsibility for any damages, etc. Just call the hotel(s) and ask what their policy is on this.