Everyone's likes and dislikes are different. If you've never been to any of the ports before you might want to choose excursions that give you an overview of the destination. If you have specific interest such as snorkeling, zipline, etc. you could pick a combination of siteseeing in some ports and activities in others. If you see an excursion that you know you would really like to do and would be disappointed if you missed it, then go ahead and book before your cruise. Those you are not sure about, then wait and book onboard. They will have port talks the day before the port which will go more in depth on the destination and help you make a more informed decision. Some people like to book excursions on their own to save money. However; being your first cruise and unfamiliar with ports you may feel more comfortable booking with the ships excursions.