If you don't give us a clue as to the ship/cruise line we cannot help. Most cruiselines that I use have a link to their (overpriced IMHO) shore excursions from their website. Have you tried looking there.
You can usually book by going to the cruise line web site and entering details of your reservation, however, as you don't say which cruise line you are going with that's about as much of an answer as you are likely to get! Could you throw us a few more crumbs of information next time please.