oh...don't do this blindly. It's cool that you ask, but those questions are scary. If you have the chance to get " the conscientitous marine aquarist " by bob Fenner or " the new marine aquarium " by M. Paletta . I have. But i bought them a little late a made some errors that i could avoid.
Usually you mix your salt in a bucket or container. IF there's nothing in your tank ( maybe except dry substrate ) you could do the first salt batch in the tank. You fill you tank almost completely with water ( like maybe a couple of inch under the top ), depending of much LR you have. If you wanna put 30g of water and you end up with 34g when the rock is in...it's gonna be harder for you to have the salinity you want after that when you need to put more water ( mixed with salt not in the tank )
In a way, it'll help you pratice with your hydrometer/refractometer but it's not safe when you begin ).
Follow the instructions for mixing the salt. I mix 2,5 cups in a 5g water. Don't put all the salt in your tank blindly. When you check your salinity level, be sure that the new water is at the same temp as the one in your tank .
You may wanna pass on tap water. If you have some $ left, try to get a reverse osmosis unit to filter the water. It'll save you headaches and money.
Unwanted algaes everywhere is not fun.