neupane00 - I understand that you are new to the SW hobby. I would suggest that you do a lot of reading up on keeping a SW tank, especilly if you plan on having a reef tank. I say this because this hobby can cause a lot of headachs if you start out by trial and error. There is a little bit to learn. If you spend a few months reading first, things will be a lot smoother for you, and prob. less expensive. Buy a good how too do saltwater tank book. They are very usefull for begining and later on as reference.
You have to be very carefull of what is told to you by the local fish store. This is a very good forum. Read a lot of other articles and learn from them.
The answer to one of your questions - also i assume u add the salt and other chemicals (what other chemicals?) - You add salt to the ro water on the initial filling of the tank and water changes only. When you top off evaporated water, you add ro fresh water only. No salt. When salt water evaporates, the salt is left behind.
You DO NOT add chemicals. NONE. The salt mix will have every mineral that is needed. Once you do more reading and become more experienced, you will learn that you might have to whats called dose, depending on your reef load and type of corals. Look through the articals in this link. This will give you an idea as to what is involved. I am not trying to discourage you, just trying to help. Good luck.
By the way, I have had fresh water fish all my life, and when I started SW 1.5 yrs ago I was clueless. I read and researched equipment for about 6 months before I filled a get my feet wet 10 gal. tank. 75 Gal. soon. I am so glad I went that route. Check out these articals
http://archive.reefcentral.com/forums/showthread.php?s=&threadid=102605
http://www.reefkeeping.com/issues/2005-05/rhf/index.php