How do i look after tropical fish? salt water or fresh water.?

my mum broght a new house and she asked for the fish tank too. we filled it up brought a filter/pump, heater, fake plants, rocks, boat and bridge and off course tropical fish. ( lampeye tetra, 6 ) and i think they are fresh water. but a few hours after buying the fish they died. the shop keeper said the water had to be 26 degrees celcius. and it was set at that. i used water from the tap to fill the tank up. the stones at the bottom were natral but were meant to be used for a different purpose. and the bridge and boat where japanese but could also be used in fish tanks. i have really 3 questions.Q.1 how do i look after tropical fish? ( i.e what will i need to make sure they stay alive ) Q.2 what do you think me and my mum did wrong? Q.3 do the things that go in the fish tank need to be specially made to be in the fish tank? ( i.e like i borught the stones from a hardware store, meant to be used for path ways. and my mum brought a boat and bridge from a market meant for ornaments )