Welcome
Coldwater fish are fish that live in cold water, Freshwater fish are fish that live in freshwater. The coldwater and freshwater section names are a bit misleading :lol:
Do not get goldfish. That tank is way too small. I know the petstore says its ok, but this forum will tell you otherwise.
Get a Siamese Fighter, but you will need a heater for the tank
The silica sand wont need replacing each clean. You just use your water siphon, a length of clear hose, to suck the poo etc off it, just don't put the hose close enough to suck up the sand. You do not need a pH tester. Only if you want, but I personally never measure my pH. You can take a water sample into your local animates for a free testing.
$10 container of Bettamin from animates (or probably cheaper at hollywood fish farm) will last you a very very long time.
You can get Betta's (Siamese Fighters) off a few breeders on this forum.
If you go to the "user control panel" and go to your profile. You can add your location which will be useful for us when we suggest breeders or places to buy stuff