how do you set up an aquarium and do fresh water fish need heaters?

Answer 1

That's way too much info to try to cover in an answer here - there are entire books on this subject!

I can direct you to some good websites where you can find the information you need though:
http://www.firsttankguide.net/
http://www.fishlore.com/
http://www.peteducation.com/index.cfm?cls=16
http://www.wetwebmedia.com/

Some freshwater fish do need to have heaters (these are called tropical fish and should be in water temperatures 76-80o [sometimes higher]), while others (coldwater fish which live in water less than 76o) don't need them. If you look through the species profiles in the links above, you'll find temperature ranges for the common aquarium species. Generally, goldfish, white clouds, some barbs, and some loaches are considered coldwater. Others like danios, guppies are at home in the high 60s to high 70s range. When planning your aquarium, you should look for species that have similar temperature requirements.

Answer 2

As the first poster said there is simply too much to answer in that question.

Here is a great article on starting your first tank: http://www.aquaria.info/index.php?name=News&file=article&sid=322

And yes, tropical fish do require warm water, so in most climates that means you need a heater.