In a brand new system, you can fill the tank, then add the salt. Once the tank has been set up, you will need to mix the saltwater according to the instructions, in a food safe container. I recommend a Rubbermaid Brute trash can as they are food safe and most often used in the hobby. Allow the water to sit for 12-24 hours with a powerhead and a heater. After that, test the SG and adjust as necessary.
One thing a lot of people fail to identify is that you need to add the salt to the water, not vise versa. In other words, do not measure and add the salt to your mixing container, then dump in the water. First, fill the container with the desired amount of water, then add the salt. Water can only hold a certain number of ions (salts) in solution before they begin to precipitate out. By adding the salt first, you far exceed that number, and precipitation begins immediatly. By the time you get the container filled, it would look like a snow globe.