A big mistake that many new salt keepers make is on topping off the tank due to water loss due to evaporation. When doing this do NOT add salt to the added water as water evaporates but salt does not. It is different when doing a water change as you are actually removing water AND the salt the water contains. In this case you do add salt to the new water as you are removing salt along with the removed water. I suggest getting a hydrometer to test salinity. A simple unit is cheap and can save you from killing the tank.
As to the substrate it has been many years since I've done salt but, if I did today, I'd still use the same substrate that being aragonite. While there well may be exceptions most salt fish want a high PH in the area of 8.0 and aragonite is a natural PH buffer that will raise the PH level. The problem is that most water sources you are likely to use for water changes are going to have too low of a PH.
Now as to fish population I suggest that you go with simple and hardy beasties. At least until you gain experience I would suggest such as damsels, clowns, blow fish and some anemones for the bottom. These are all VERY hardy and forgiving as to water conditions.
What I've said is just general stuff. If you take this on we would need more info such as the tank size, PH of the source water and if the tank would be straight salt or a 'live' tank with such as live coral.
Since you are new to this keep it as simple to start as possible and do NOT trust a fish store to give you good advice. Oh, and if you ever see what looks like a sick fish, NEVER add antibiotics to a salt tank as it will kill the tank. A fresh water tank can have the good bacteria survive moderate doses of an antibiotic but, unless things have changed, a salt tank's good bacteria cannot. Without the bacteria colonies a salt tank is dead.