Tropical Fish The Basics: (my opinion anyway)
To start with: set-up your tank just as you did with your goldfish (cold tap water) add the water conditioner, stick the heater in turn her on and a way you go.
Heat: use a aquarium heater (which cost $20+) these will have a in built thermostat which turns the heater on/off keeping the tank at one temp (set by you) most tropical fish need around 25'c.
Fish: neons, sword tails, rocket pencils, guppies, clown loaches.
Food: a good tropical fish flake, with some frozen or live food 1-2x per week (water flea, bloodworm, brine shrimp etc.)
Water changes: remove 20% of the tank water per week and top it up with fresh stuff.
Note: the only main deference between tropical and cold is the temp.