Fish are finding there way to peoples aquariums because they look prettyor they're on sale. If you could inform yourself with a bit of knowledge you can save lots of money and grief in the long run. Lets talk about a few different kinds of fish and what they need.

Neons: minimum of 6 in a school. love to swim near the bottom and hide in live                                 plants, no seashell or coral decor, can be kept with other mild                                                 mannered fish like black neons, glow lights, harlequin rasboras,                                             corycatfish, small plecos, otocinclus, will need something a little                                             larger to keep them in a tighter school like an angel or two.
           soft water, driftwood, plants
           
South American  minimum 66 gallon tank (some can get quite large), hiding
       Cichlids:       places or at least plant coverage, keep with sevrums, jewels,                                                  (oscars if you must), parrots, rafael catfish, plecos, clown                                                         loaches, congo tetra and many more
                           soft water, plants, driftwood
   
African Cichlids: minimum 48 gallon tank, many many many rock caves, coral                                                   or other hard water substrate, keep with other agressive                                                          africans of similar agression, lace cats, large plecos, clown loaches,                                     mild africans can live with some rainbows but the                                                                      substrate should not be completely coral.
              hard water, no large driftwood, few plants (if they don't get eaten) 

Teacup Sting  minimum 4' tank, large tree roots to the water surface                                               Ray:        sand substrate, keep with other like rays (obviously you'd need a                                            bigger tank for more rays), large plecos and a few other mild                                                  medium south american fish, very timid
                       soft water, sand or very fine gravel, driftwood
                       
Filters are the most important part af any aquarium. Why do people skimp out on that. Always go bigger, you'll be very happy in the end when debris isn't floating aimlessly around in the aquarium. Auqa Clear 50 and 70 filters are good for anything up to a 33 gallon. Eheims and Fluvals (canisters filters) are great for anything above that. Do not buy filters that have a slide in filter component where the carbon and sponge are one in the same. If you discard the replacement cartridges, you remove the beneficial bacteria that keeps the tank healthy Aqua clears have plently of room for additional bacteria like bio balls and ceramic rings, but if you have to use meds or do not want to use carbon with live plants, then you don't have to use it. It simply has layers of media instead of a single slide in unit.
Heaters should be submersible not the hang on the side kind. If you do a water change or your water level goes down with evaporation, you shouldn't have to worry to much.
Lights should be bright like the sun. Use Power Glows or compact flourescents for a true bright white light that brings out the great colours of your fish, unless it is recommneded by a knowledgeable retailer regarding the fish's unique needs.
Glass scrubbies of the white variety are the best. I get mine from auto body places (thanks Lissette). If you really have to razor blade your tank never use the same one more than once-but I don't like to use them. A special note to Jack and Angela, unfortunately you learned the hard way-NEVER use SCOTCH BRITE PADS!!!!!!!