At the shore excursion desk there will be a sign that will tell you when they open. If you want to make sure you get the tour of your choice then be there. You can also book from the TV on many ships but I always prefer to waste a little time and stand at the counter to make sure they get my choices.
We did ours as soon as we got to our room. It was VERY easy and I recommend doing that to anyone. One reason is because they sometimes sell out very fast and if you do it right away, you can go have fun and not worry about anymore.