You can check-in as early as you'd like. The front desk will issue your room ID's and any admission media you need, you'll get all of the resort information. If your room is ready (which depends on occupancy the prior night, when other guests have checked out and housekeeping schedules), you will get your room number at that time. If the room is not yet ready, you can call the desk from any WDW phone (they will provide the number) and check later to get the room number.
We have, at times, had our room by 10 AM- at other times not until after 4PM. In either case, you can check-in when you arrive- you just may not be able to go to your room.
Good Luck!