here is my advice for ya since it seems to be your first setup...
dont go into salt water since it seems as if you have not yet done fresh water... Its vary expensive and hard to maintain... start with fresh water and see how that goes then convert if need be.
If your looking for advice... find a cheap setup on craigslist or a local paper... you can usually get them as a complete setup for around $1 a gallon....
what you need depends on the type of fish you want to keep. You want to create a natural environment for them as they would have where they are native....
most fresh water setups are going to require mechanical filtration, biological filtration, heat, airation, lighting, chemicals, testing supplies and many other little things.
Mechanical filtration is a normal filter.... you best choice would be a canister filter and combo that with hang voer the back type filters but in a small tank like 50 gallons a couple of large over the back filters would be fine..... there are many other types of mechanical filtration but these are the two most commonly used now days.... Dont bother with under gravvel filters or sponge filters as a beginer.....
Biological filtration is basicly bacteria that grows and lives in the water, substraight(sand or gravvel) and basicly anything else the water is in contact with. This is established by letting you tank cycle(sit and run empty or with a couple cheap fish) for a while.... usually takes a few weeks to level out while your nitrate, nitrite, amonia and PH establish themselves....basicly this will come with time... just dont go buy a whole setup take it home set it up and toss a bunch of expensive fish in there cause they will die without a good biological filtration....
Heat is easy... try to get two small heaters that will equal around the same amount of wattage as the tank has capacity in gallons.... so 50 gallon tank get a couple 25 watt heaters and set them at opposit sides of the tank to create even heating and no hot spots....in a small tank like a 50 gallon you could get away with a single 50watt heater though... You also should have several thermomitors.... you can get cheap digital ones for like $7 at most pet stores.... they are the best...but also a non electronic one with a suction cup works well on the inside just keep it away from the heater....
Lighting isnt too important in a fresh water setup just use what you get with the tank you buy... you dont need much in the way of lighting for a small fresh water setup.
airation is exactly what it sounds like... get a air stone and put it under the sand or pebbles you use as your substraight and let a air pump air through the water... it isnt really putting air into the water as much as its keeping the water moving and thus letting it take oxyggin off the surrface....
get some test strips they are like $10 they have all in one test strips everywhere to test nitrite,nitrate, amonia, ph and hardness... m,ake sure your tanks levels are stable at what the fish you keep like....
also try to keep a natural looksing environment for the fish... they arent really happy with colorfull rocks and sands on the bottom and mean looking sharks bubbleing at the mouth and fake ship wrecks.... and fresh water fish dont like corals... remember fish keep away from bright colors cause thats a sign to keep away in nature... if you want them to swim around alot and be entertaining the best bet is a natural looking tnak setup... They will develope coloring and be more at ease with this type of setting.
Fish love places to hide so lots of rocks and plants(pref real) are welcome by most fish... Be carefull with the rocks you put in your tank cause most rock will harm fish and the water....i know it sounds weird but dont go to a lake or river and use those rocks...you will probably kill your fish off pretty fast... a good rock used by many people and is considered to be safe is "lace rock" or also grannite.... you can purchase these stones at rock yards for really chepa take them home clean them and boil them for 10 minutes.... then they are ready for a tank... at around 17-50 cents a lb thats about $5 per pound cheaper then a pet store... good luck with the fish...