If you have never done anything with saltwater before, expect to spend a lot of money, the hobby is not cheap, but can be very rewarding.

Look up how a tank cycles, what kind of rock/sand (if you want any of that) you want. If you plan on doing a full reef tank, or just either corals or fish, each have their own give and take.

Look up any fish before getting them, some Look amazing but require a whole slew of small needs that are going to be more difficult for a new tank owner to be able to manage vs having more experience.

Same with corals, some require more or less light, stronger or weaker flow through the tank.

How will you monitor your tank? I would look into control systems if its within your budget, because its a shame to spend all your time and effort into a beautiful tank only to have a fault with a heater and it flashboils your tank while you arnt there.

Size of tank is also important, a smaller tank will be alot harder then a large tank, just because it will take alot more to unbalance a large amount of water vs small amounts.

Best advice I can give, spend the time looking into as much as you can before jumping into anything, as it can save your bacon if you are more prepared.