There are a lot of different opinions on witch tank size to start with, usually its the bigger the tank the better. I personally think a good tank size to start off with is a 55 gallon. Smaller tanks are a bit more difficult some people say. The basic things you need to start with is a test kit, salt, hydrometer (salt measure), filter, live rock, live sand (oppitional). The live rock I think is a good thing to have because it comes from the oceam, and it is great for biological filteration, witch means the rock acts as a filter. Also on the rock you'll get many different things growong on it like mushrooms, plants, different types of algea, ect. It can be a bit pricy but its worth it. Also live sand is a good thing to have, like the live rock it has many ting organisma living in it and very benificial to your tank because it helps reduce deadly ammonia and nitrates in thew water, actually the deeper the sand the better. A DSB (deep sand bed) is what really helps with the water parameters. There is also crushed coral but I do not reccommend this because it does not help with the nitrates and you have to clean it manually, with live sand the inverts clean it for you. As for a filter I just use a wet/dry bio wheel filter and it works great for me, but there are also sumps and refug-somthing forgot what its called, these cost a lot more than the wet/dry filters though.
I reccommend using RO or DI water, this water IMO is better to use in your tank. Your salinity level should be around 1.020-1.025. After salt water is added then you can add sand rock whatever, you do not have to add rock when you first set it up it can be added when ever you want. You must let your tank cycle before adding any fish or inverts. The cyclce is when, well I'm not that good at explaining the cycle but its when the nitrite, nitrate and ammonia levels go up and down. In saltwater to keep a healthy tank you must be patient. I know some new people will go out and buy fish or somthing within the first week or so, a definite no no in saltwater. IMO you should add fish very slowly to your tank and wait a while after your tank cycles. The cycle can take from 2 weeks to 2 months, each tank has a different cycle period, you will know when your cycle is finished when all your levels are fine and you will see a light brown coloered algea covering the sand, rock and glass, this is called diatoms and a sign that your cycle is nearly finished. You can wait for it to go away or you can add inverts, this is the time IMO when its safe to add inverts. Your water parameters should be
Nitrite: 0
Nitrate: 0-10
Ammonia: 0
Ph: 8.1-8.4
Calcium: 400-500
You should have a clean up crew in your tank (inverts), such as snails, aquatic hermit crabs, starfish, shrimp ect. These will eat uneated fish food or detris in your tank wich is why they are called a clean up crew. Note: inverts are sensitive to salinity and drastic water changes so be careful when adding them and hat your salinity stays in one spot.