I hope my post helps since I know starting this is SO overwhelming! I read on RC for about a year before I started into this hobby. I originally was going to do a 55g (which i finally am now) but with budgets in mind and an impulse buy at the LFS I got a 12g Aquapod- probably very similar to your tank just a tad bit smaller.
I knew it would be a little more difficult to keep than a larger tank- smaller amount of water = more things can go wrong. For an example, lets say you have one fish. If one fish poops in 12g of water, its going to probably stink a lot more and for longer than if a fish poos in 55g of water - because of things like that little tanks take weekly water changes of slightly larger proportions (20-30% maybe? you will feel out what is right for your tank) ..
That aside, test kits test kits test kits. I cannot emphasize this enough! And something to measure your water's salinity (how salty it is)... A hydrometer will get you by, a refractometer is better.
Now I recommend at least once a week checking your salinity (1.025 give or take a .001 or so), pH (8.2ish is good) on a regular basis.
While cycling you want to test for Ammonia which will spike first- then bacteria from the LR will eat the ammonia and produce nitrite - that should spike second while the ammonia begisn to fall and go to 0. The nitrates may spike ... Once Ammonia, Nitrite are 0 and Nitrates are <10ppm, it is probably safe to put in a clean up crew.
I kept my tank goin for a bout a month and had cured LR from the LFS (in a small tank its so easy to transport 10-15 lbs of LR from the store) and never saw a cycle despite testing every other day for those things.
If your water isn't showing any signs of anything going on you could *try* to keep the fish, but if you're unsure go ahead and let the LFS hold onto him .. sometimes you will only get a mini cycle or will need something to start your cycle. You could add a little bit of flake food or a cocktail shrimp (remove shrimp once ammonia shows) to the tank to decay and create a catalyst for the ammonia.
But like I said once everything is 0 you're good to go. I recommend maybe 1-2 nassarius snails, 2-3 margarita snails and maybe 3-4 hermit crabs (the mexican ones are good but the full name eludes me right now)
Let your tank adjust to these critters and if you feel you must feed them just put a tiny amount of food in the tank.
In a week or so you could probably then add your clown if that is what you want to keep... you could also probably put in a shrip like a skunk cleaner shrimp- really cool friendly guy that'll hop on your hand if you let him.
The *best* possible course of action is to just go slowly. There's a lot of unseen biological and chemical changes that take place in a little tank like that and even the larger tanks too. A huge ocean can handle them but we need to pace ourselves despite how exciting everything is!
Also for your LFS, call the person an idiot or not but the burden of responsibility of our tanks falls on us, the aquarist. If you can do your homework and avoid impulse buys before deciding on a purchase - or to be able to confirm or deny a claim made by a fish store- not everyone is right all of the time and no LFS is equal - than you could still have a pleasant experience.
On a personal note, if you can, I would avoid recommending large chain pet stores unless you have a truly reputable one in your area. They tend to cater toward the masses and unless they have very knowledgable staff, chances are what you buy from them won't have a good chance of making it in your tank and you're better off starting out with happy, healthy specimens.
Truly hope this helps, and happy new year!
Keep testing for ammonia, nitrite, and nitrate and hope the cycle is quick. Normal cycles can take a month. Only after you have NO trace of ammonia or nitrite then you are safe to add a cleanup crew. I'd then even wait a week or so. You are in the right place as far as research goes. Keep reading and learning and you'll have success. Remember, your LFS needs to make their monthly goals. Take what they say with a grain of salt. 
I'm done going to that fish store. 
