Cabins - Anything midship is best for first time cruisers... please ask them not to upgrade you if you are able to reserve a midhip cabin.
Must dos on the ships... that is really up to what you like to do... personally, food is high on my list so the Pinnacle Grill is a nice evening's event. Take in a few shows... the comedians always seem to be decent.
Do not skip[ out on the formal evenings. They really are quite nice and a chance to 'glam up' even just a little.
Briefings on your ports of call are usually interesting and informative... always better however, after a glass or two of wine! Remember, on HAL, you can bring wine on board!