oooh ok so i dont need to buy a big mechanical filter or skimmer ? just the normal filter used on tropical tanks ?
No. You don't require an external mechanical filter like those that are commonly used with freshwater setups, provided you have sufficient live rock in the tank. As mentioned before, Live rock provides the filtration for the tank. Its very porous and as such, provides ideal breeding grounds for the denitrifying bacteria. It is expenisve, but it looks great.
Basically a run down of things you will need for a marine setup.
Powerheads. (Water pumps that circulate the water. These are needed to keep the water flowing around the live rock)
Heaters. (Always have a backup incase one dies)
Protein Skimmer. (These aren't absolutely essential, as pollutants can be removed with frequent water changes, however they are highly recommended)
Live Rock
Sump (not essential but highly recommended again)
Calcium Carbonate substrate (aragonite sand. A nice deep sandbed will also provide a home for aerobic and anerobic bacteria too)
Marine Salt
UV Sterilizer (not essential but worthwhile investing in at some point to kill bacteria and parasites)
T5 lighting or Metal Halides if possible. Good lighting is a prerequiste of keeping corals.
Water testing kits for - Calcium, PH, Alkalinity, Ammonia, Nitrites, Nitrates, Phosphates and a million other things.
Refractometer for measuring the SG (salt content in the water)
Marine salt
Reverse Osmosis water (purchase from LFS, although in the long run its better to get an RO machine and produce it yourself)
To answer your other question, the specs you posted before are a FOWLR tank.. not Fish Only. FO literally contains just fish and is usually powered by an external filter or sometimes a deep sand bed. FOWLR is a tank with fish and live rock, but no corals etc as that would be classed as a reef setup.
Before you rush out and buy anything, take the time to research all the different setups that can be utilized to keep marine fish. For example, A fish only tank with just base rock can use regular tap water, that has been dechlorinated and has Marine Salt added to it. Although this often has high nitrates and phosphates, marine fish aren't as affected on the whole by these "contaminents". However, if you was to go this route, you wouldnt be able to keep invertebrates or corals. It does work out cheaper, but greatly limits your stocking options.
As mentioned before there are other filtration methods, from mechanical (external filters) to live rock, to deep sand beds or hybrids of the latter two. Personally I'd recommend going with a FOWLR setup first. Get to know about the water chemistry and getting it right. A sump is a worthwhile addition to have too, as it increases the overall volume of water, thereby reducing pollutants and making the chemistry more stable, but it also means that heaters and protein skimmers can be housed out of sight in the cabinet below. The sump can also contain a refugium, a chamber in the sump which has calm water in which macro algaes (seaweeds) can be grown. These absorb nitrates and phosphates and improve the water quality. When keeping corals, water chemistry is paramount and there are a lot of parameters to check and maintain in order to keep a healthy reef.
Try reading through some of the setup threads to see the different types people have used and equipment involved. The decide what you want to go for, then think about what equipment you will need and how to set it all up.