I'm far from an expert, LOL. I've just setup a bunch of tanks so I kinda know what ya need.
You'd need at least 30lbs of rock, live or base(dead), 30lbs or so of sand, a powerhead and salt. You also should use ro
water which means either like $70 for your own machine or buying it from your lfs
or a supermarket water machine. You''ll need a heater just like in fw
so thats no different. You can use your current filter setup too.
If you have the money a refractometer $30, or a hydrometer $15 for testing salinity.
Thats all I can come up with off the top of my head.
Hope that helps!