Having cruise on Explorer last year, so far our best cruise ever, we did have a D7 cabin with a balcony and absolutely loved the cabin and location. Have heard a lot of nice things about the Atrium cabins as well, only major noise would be when they are having one of the parades go through, but hey, you get a great seat for that. Also, be sure to pull the drapes when changing etc., as the same cabins across the way can see into yours very easily and same for you.
You may also want to do a check with the "bargain Finder" here at Cruisemates and see if you can find any better prices for when you want to go, once you are comfortable with a travel agent and their services and of course the price, go ahead and book it, especially if there is a certain cabin or catagory that you really want.
Once you post in the bargain finder, travel agents will e-mail you with what they have on that specific cruise, its very easy and hassle free.
You can also get a lot of good information and tips by reading through the section for "first cruisers", should also answer a lot of your questions before you book:
Just make sure you have the ship layout and can see where the available cabins are located, I personally need to have a window, but best yet a balcony, but I think the atrium cabins are also nice to have, its really a personal choice.
If you have any more questions, feel free to post here as often as you need!