The first thing that I will recommend is for you to read a few books such as Scott Michael's Marine Fishes Pocket Guide and The New Marine Aquarium by Michael S. Paletta. Also make sure you do a bunch of research before starting.
So your tank is around 23 gallons correct? if so then it would be a great size tank to start with.
Live Rock. A good amount of live rock for a tank would be around 1lb per gallon of water, as that would give you plenty of live rock. Some people including me use another method where you add 80% base rock to your tank along with 20% live rock ontop and around the base rock, as eventually the live rock will seed the base rock. This method also saves you money.
Sand. There are many choices for sand including live sand, aquarium sand, and you can even use playsand. For my tanks I use Carib Sea white sand as it is a nice bright white, and is reasonably priced. Live sand imo is a waste of money as overtime the live rock will seed the sand anyway. I personally go for a 1-2" sandbed in all my tanks as I find this depth is enough to make the tank aesthetically pleasing, and still make maintenance easier. If you do decide to go with a sandbed then I would recommend to add many detritivores to your tank as they will keep the sand stirred up. Nassarius snails do a great job and moving through the sand stirring it up, as well as various pods, and worms that will do the same.
You can actually buy detritivore kits from
http://www.inlandaquatics.com/, although I have never bought from them people do recommend them. I think Liveaquaria might detritivore kits also.
I'm guessing from your description that the tank is an all-in-one kit meaning tank wiith built in filter, and lighting correct? if so what type of filter and lighting does the tank come with?
Corals. Xenia imo isn't the best coral for beginners since it's it's propensity for "crashing", and then people start freaking and messing with their tank, dosing Iodine which just makes matters worse. Since this is your first saltwater tank I would recommend you start out with easier low-light corals such as Zoos, and Mushrooms.
Flow. Flow is very important in a saltwater tank, and the right amount of flow is required for a successful tank. For a simple FOWLR 10-15x turnover, Softies need 15-20x turnover, LPS need 15-25x turnover, and SPS need 30-40x turnover. Also having two powerheads is better than one as you can point each powerhead in a different spot in the tank so you get adequate flow throughout the tank.
Stocking. Two Clownfish such as True Percula or Ocellaris would be fine for your tank with adequate filtration. When adding you Clowns add them either both at once or if you add one, and then want to add one later choose one that is smaller than the current one. What Shrimp were you thinking of adding? Also Turbo Snails get quite large around 4", and plus there are other cleaners you can consider, such as Nassarius Snails, Margarita Snails, and Cerith Snails. You add more Hermit crabs if you want, but make sure you provide extra shells for them so they don't fight.
I think I covered all your questions and more, but if you have anymore questions feel free to ask. :)