Hi Racing to go,
You may have a cabin assignment by the time you leave, if you do, simply write that number on the tags. I'd call your travel agent before you leave and see if there is a cabin yet, if not, when you get to the pier, check with the people there before you turn your luggage over, they will have a cabin number and then direct your luggage to the correct cabin. In that case, just leave that blank until you know the cabin you're in.
What ship are you on and when are you cruising?