Hi and welcome to the forum
Saltwater tanks are similar to freshwater tank when it comes to cycling. Add a source of ammonia, let the filter bacteria develop, when it's done, add some fish. It normally takes around 4-6 weeks to cycle an aquarium but can take longer depending on temperature and other things.
The ammonia source can be bought from most pet shops. They have instructions and you usually add x amount each day until a certain level is reached, then monitor.
The tank can have a thin layer of beach sand or shells on the bottom. You only need 1/2 an inch of sand or shell.
Add some limestone rocks to the tank to create hiding places and make it look nice. You can add some plastic plants too if you like.
Fill the tank with sea water (either natural from the ocean or artificial sea water made from marine salt and dechlorinated water). You can buy marine salt from most pet shops or online. Marine salt needs to be made up at least 24 hours before using it in an aquarium with livestock (shrimp, fish, corals, etc). The easiest way to do this is filling a bucket with tap water, adding a dechlorinator to the water, and then adding the marine salts. Aerate the solution or use a water pump to mix the salt and water. Use a hydrometer to check the salinity (salt level) in the water 24 hours later and either add more salt if the salinity is too low, or add some freshwater if the salinity is too high. Let it mix for a few more hours and when the salinity is correct, then use it in the tank.
If you have nitrates in your tap water, then get a reverse osmosis unit to make clean water or use a nitrate removing substance to get the nitrates out of the tap water.
If you collect natural sea water, try to get it from areas where there aren't many people or boats and there isn't an oily film on the surface.
You need a hydrometer to measure the salt level in the water. There are 3 main types of hydrometer.
The floating glass hydrometer. These floating in the water and have a scale on the glass. You look at where the scale is compared to the water level. They are reasonably cheap but can break it dropped.
The plastic chamber hydrometer. These are a thin square plastic container with a plastic lever/ needle in. There is a scale on the outside of the container. You fill the hydrometer with salt water and see where the needle points. These are quite good and last for a long time. Their drawback is they only have a limited range on the scale so the water is really fresh or really salty, it won't be able to tell you the numbers, and will simply point the needle to too much salt or not enough. This is my preferred type of hydrometer.
The refractometer is the most accurate out of the 3 types of hydrometer. They are more expensive than the other 2 types but will read salinity in ranges from virtually none to double the strength of seawater. They should be calibrated regularly (see instructions that come with it to do this), and can be damaged if you drop them.
If you live near a clean beach, go there and grab some sand, rock and water. Take it home and put it in the tank. Cycle the tank and then add some fish. Just make sure it's legal to do so because some authorities get stroppy if you grab a bucket of sand or some rocks from the beach. It's alright for little kids to do but not an adult.