Congrats on your 20th! Great milestone!
That's a pretty broad question. Whether you prebook your excursions or wait until you get there will depend on what you want to do. If you are interested in things that have limited space and/or tend to be popular, by all means prebook them. In Alaska, you can sometimes book them with third-party companies, rather than with the cruise line, for less $, but if you have a short time in port (1/2 day, for example) or are planning to be a ways from town - for example, going out whale-watching or on a fishing excursion - I'd recommend doing it through the cruise line; if you get back late, and are booked through the cruise line's excursion desk, they will wait for you, but if you don't, they won't, and you'll have to fly to the next port at your own expense. (Can't rent a car and drive - Inside Passage towns aren't connected by roads, for the most part.)
Once onboard, if you find, after prebooking an excursion with the cruise line, that it's not quite what you want, you can cancel it (usually up to 24 hours ahead of time). You won't get a cash refund, but they will usually offer you credit toward a replacement shore excursion.
Check with your TA or with the cruise line if you have specific questions about the excursions or about booking procedures. Have a great cruise!!