Welcome to Cruise Critic!
As a newbie, select an itinerary that appeals to you. We loved Hubbard Glacier and most people also love Glacier Bay.
We book excursions from companies onshore, typically not through Princess. However, be aware of Viator as they are brokers and we know many people who had there excursions cancelled basically the day before.
I know nothing about the Royal. Many love that ship and many don't.
Princess does not care when you book. It only benefits you because you can select your cabin.
The WiFi works great for us. We love booking an aft balcony with the larger balcony. Balconies are not created equal. The captain does a 360 so everyone gets great viewing, however, you will be wanting to move around on the ship and not need to stay in your cabin.
There is also a ton of information on the Alaska forum.