See if I can clarify.... (if some of seems too dumbed down, I mean no offense, for just starting out, you are really getting the handle very quickly).
The reason you may have been told not to do a water change, is because your filter's are fresh and haven't "cycled." They need time to build nitrificating bacteria. Nitrificating bacteria is what makes are saltwater tanks possible. They convert ammonia and nitrite into nitrates. Ammonia and nitrites come from left over fish food, fish wastes, etc. Ammonia and nitrites are also poision to fish, but their amount in the tank has to be high enough, in order for the bacteria to form. However if the levels get to high, they can and will kill your fish. That is why a normal 'cycle' is done without fish. It will be sort of a juggling act for you. Trying to get your tank to cycle without the levels getting to high.
It really sounds like you want to become part of this hobby. At this point, the best thing you can do, is get yourself some good test kits. There is little we or the LFS can do if we don't know you're water parameters. You already have something to measure the salt level (salinity), the basic test kits you need right now are, Ammonia (NH4), Nitrite (NO2), Nitrates (NO3), and pH. These last for a while and are worth every penny if you plan on staying in the hobby.
It sounds like you have two types of chemicals and like you know both, but anyway, one type is more or less, nitrificating bacteria in a bottle. They will quickly consume ammonia and nitrites. The other is indeed used to treat tap water, mostly to get the chlroine and other chemicals out. If you just plan to keep this fish or only fish and no corals or anything, using tap water is going to be fine. Many "veterans" here may or may not admit it, but they use tap water as well.
You want to use the chlorine stuff when you do a water change. Add it to the water before you mix the salt in, and definitley don't add directly to the tank (just making sure). For the Start-Up stuff, read the labels and do whatever it says. That and water changes should keep everything safe. If you are going to get test kits, we can go from there. But for now do the suggested water changes, every 2-3 days, 5gals, and add in the correct amount of chemical according to the back of the label.