Here's some standard sizes if you don't know what tank consists of how many gallons.
http://www.anapsid.org/resources/tanksize.html

A lot of species sold in pet store don't need a heater. Only really sensitive species like discus, angels, neons need heaters.

Next, read up on how to cycle an aquarium before you get fish. http://www.aquariumadvice.com/articl...q=2&fldAuto=21

As for fish, I suggest you stick to fish that stay under 4" as adults, unless you are willing to upgrade to a much bigger tank in the future. Don't buy something because its small and cute as a baby and then neglect its needs as an adult. The stocking guidelines you hear people talk about (1" per gallon) are meant for the adult size of tropicals. Coldwater fish need even more room to grow.