You really need to have sand & rock when you start. Adding later causes tank to cycle again. The sand & rock is where the beneficial bacteria live (they eat the waste in your water).
A great beginner book i bought was saltwater tanks for dummies.
Fish only tanks are easier....but still not what I would consider easy or cheap compared to fresh water. The expense of a fish only salt tank can still be substancial.... There are several tests that need to be done weekly, weekly water changes, etc. Medication to keep on hand in case they get sick. I consider a $20 fish cheap for saltwater ($20 for freshwater is alot).
If the time & cost doesnt bother you....learn as much as possible before you start. Dont buy cheap equipment, it usually works poorly if at all. Look into buying good quality equipment (buying it used when you can helps save $).
The fish store guy saying it isnt hard... Is so not true. Sadly the workers at major chain petstores dont know much about salt tanks. Check out your area for a predominately saltwater store, see if there is a local saltwater club. You can get lots of info, help, and even good buys on used eqipment.
Lastly, starting a salt tank ia usually a slow process. You cant add saltwater,sand, rock & fish & presto instant tank. You start with sand & rock (generally 1-2 lbs of each per gallon of tank) then allow the nitrogen cycle to run its course (can take generally 2-4 weeks, sometimes longer) before you get to gradually add fish. Cycling the tank is when the good bactetia grows to take care of waste. Each fish added adds to the bioload (waste production). You add them slowly so the bacteria continues to grow and isnt overwhelmed by the waste (which causes another cycle to happen).
Must have tests:
Salinity (refractometers are easy to use & worth the $$)
Ph. (required chemical buffers to adjust it as needex)
Amonia
Nitrate
There are many others....but these are must have before you add the first drop of water kind of tests.
Basic equipment:
Tank
Heater
Powerhead
Filter system (canister filters are ok on sm tanks, but require weekly maintanance too) (sumps are a sm tank that works like a canister filter only it uses biological processes instead of filters)
Protein skimmer (not a must have, but goes a long way in helping water quality).
I hope I didnt scare you. I just want you to have a clear understanding that saltwater tanks take time, money & patience.
Good luck!