Hey, I have 3 saltwater aquariums, 30gal, 30gal, 70gal, and work at Petco as an aquatics specialist.
Basically, you can use the same tank for freshwater, as a saltwater tank, it makes no difference. It is about 2-3x more expensive than freshwater, but worth it.
As far as water goes, you can buy your saltwater from a LFS (local fish store) already made, or make your own with a RODI unit, I use a 5-stage RODI unit. You just plumb it to your water line, turn it on, and it makes a steady stream of RO water, basically just pure water. Mine makes 75gal/24hr, so I fill a 30gal garbage can with water, then I mix in aquarium salt, put a pump in for flow, and measure the salinity with a hydrometer.
If you're looking for the cheapest way to set up your tank, and you want a 25gal tank, do the following:
Already have the tank and stand
1. Buy a HOB(hang on back) filter and put it on the back of your tank for filtration. This is the cheapest method, and advanced aquarists use methods such as sumps/refugiums. Additional tanks that takes water from the main tank, filters it, and goes back into the display tank.
2. Buy 30-40lbs of live rock, inspect it for parasites such as aipstasia before buying. Live rock runs for 3-8$/lb depending on quality/type
3. Buy about 25-30lbs of live sand
4. Put sand in first, spread evenly, then put the live rock in and set it up how you want to. AFTER SAND, ADD WATER HALF-WAY TO THE TOP OF TANK, ADD ROCK, THEN FINISH ADDING WATER. Rock will change the water level.
5.Also buy a heater for the tank, you should only need a small 50w one. Buy a wavemaker pump to provide flow in the tank.
6. Give the tank 2 weeks to cycle. By that, I mean go through the nitrogen cycle. You can't put any livestock in the tank during this time. Buy some of the cheap strips for testing water. The things you want to look at are ammonia, nitrite, and nitrate. Once your ammonia and nitrite levels are 0, its safe to add live stock.
7. Get a clean-up-crew, consisting of hermit crabs, snails, etc. Can also get a starfish.
8. Add your fish! In a 25gal tank, you're limited to how many/what types of fish you can get. Only get ones that are rated for tanks up to 20-30gal. You will probably only be able to get 2-3 small fish, or 1 big and 1 small.
For corals:
If you're wanting to get corals, you will need the proper lighting fixture, which I can help you with if you choose to put corals in your tank. I would wait 3 months at least before adding corals.
Maintenance:
Every 2 weeks, do a 20% water change, so 5gallons. Take 5gal of water out, and add 5 gal of new saltwater.
Also, during the course of the 2 weeks, water will evaporate from the tank, leaving the salt behind. This raises the salinity level. So to provide for the lost water, you "top-off" your tank. This means adding RO WATER. WATER WITH NO SALT IN IT. Distilled/RO water is used for this. You just add enough to provide for the water that evaporates. You can do it everyday, or every 2-3 days.
Any other questions, just ask. I probably missed a few things, I'm leaving for my tropical marine ecology final in 5min lol