I would suggest getting a heater as well as making sure that you have some type of thermometer to help make sure that you have the correct tempt in the tank. There are heaters that have a built on thermometer so that you can set the temp of the water more accuratly however like any thing they can sometimes short out and it would be a safety to have a thermometer since some fish require certain temps.
Some fish that are pretty easy to keep are community fish such as Mollies, Guppies, Gold fish. Depending on the size of your tank will determine what the appropriate amount of fish that you can have in it. I would start off with one fish because if you add too many fish to fast it can throw off the pH balance.
I would also suggest talking to a fish store rep or perhaps looking up on the Internet about a type of fish that you may be considering. It would help provide much needed information such as how they breed, how to feed and what kind of environment that the fish need. Example, some fish don't like a lot of light in there environment or need "brackish water". Also, pH balance requirements are different for different types of fish. To help you on your way to having a lovely and very enjoyable tank make sure that you take the time to learn about the pet fish that you are wanting to keep. If you have any more questions please feel free to contact me.
Own five fish tanks-types of fish are Convict Cichlids, Pink Convict Cichlids, Texas Cichlids, Dalamation Lyre Tail Mollies, Neons, Platies, african brown knives, dwarf powder blue garamius.