The forum is fairly straight forward, you already have the hard part firgured out... how to write a reply. Same method is used for starting your own "discussion". The forum is divided up by interest area- each major cruise line, ports of call, and then some general discussion areas. Put cruise line specific questions in the appropriate forum group, discussions on ports and hotels go in their group. There is a bunch of really good information on this site, you can search for a topic (like search for "Galveston" and it will return all the previous posts discussing that.
Fireworks? I'm not sure, as I have never been on a cruise over the fourth.. Maybe some other cruisers have and can tell you. You may be having too much fun to care..
As for the phone calls, I'd schedule those for when you are in port, ship to shore calls are very pricey. Calls from the ports are cheaper, but check the rates. If you have a cell phone, most of the major US cell operators have operations in Mexico and Grand Cayman at least. Check with them before you go and they can tell you what the rates will be. The schedule calls for when you are in port.Leave the ship's phone number with whomever is watching them for you for an emergency use.
Have a great first cruise... You should have a blast, and next time bring the kids! It is a great family vacation too.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013