Mixing your salt in the tank for the first time is absolutely fine. Just make sure you follow the instructions on the bucket/box for the amount of cups per gallons of water.
However, when doing a water change, you will hear from most experienced aquarists that they mix their water overnight and let it sit for 24 hours. Most people get a pretty good sized trash can (a new trash can, so there is no bacteria or germs in it!!) and fill that up with their water (RO preferably). Then, they pour as many cups of salt that they need into the trash can filled with the water. It is also very important however that the water you are going to put in your tank is the same temperature as the water in your tank! You do not want varying water temperatures between the tank and the new water you are going to be pouring into the tank 24 hours later. Quick change in water temperature even by a few degrees can cause major stress to the fish (as well as ich). You can easily fix this problem by purchasing a cheap 10 dollar heater that will heat your water overnight. I use the new Won Brothers glass heater for my water changes and it does exactly what I need it to do (I believe it was $12.99). Also, many people like to buy a cheap powerhead to throw in the trash can for their weekly, bi-weekly water changes so that the water keeps a continuous movement in the trash can. This way, all the salt breaks down and dissolves, because if it does not fully dissolve, this can easily raise your salinity, and like wwfstyle said, kill your fish!
Just a few pointers... good luck in setting up your tank!