Don't you mean "Siamese"? All fish are not the same....some like it cool, some like it warm, they all have different needs in general.
Guppies are an easy beginner fish, in my opinion, but they will require a few of the basics. You must make sure your water is conditioned - no chlorine (or other harmful chemicals). The store has liquids or powders to condition the water before fish are added.
Guppies are tropical, meaning they like their water warmed via a heater. I keep mine at around 80 degrees Farenheit and they are thriving.
A tank needs a filtering system of some type. There are different kinds - undergravel, inside tank, outside tank. Do some research. The filters help keep the water "clean" and help the cylce of changing fish waste into less toxic debris. With that said, however, you still need to maintain the cleanliness of the water by doing water changes.
I'd recommend doing a little research before you get another fish. And, "yes" a Betta is supposed to live more than two weeks, although total longevity tends to vary. Bettas can live anywhere from two to six years!
Hope this helps. Keeping fish can be a very rewarding hobby/endeavor, but does require some work (like any other pet). Good luck. :)