The norm is about once a week, 20 percent of water. Do you have a syphon? It's best to have a quarantine tank taht closely matches yuor waters parameters, and put the new fish in there for a couple weeks to make sure they're healthy. that way if they ever get sick, your other fish in your main tank won't catch it, hopefully not anyway.
Here's a link about the testing kit.
Aquarium Water Testing: Aquarium Pharmaceuticals Freshwater Master Test Kit Drsfostersmith is an awesome site, and excellent customer service in my opinion. You can also surely find the kit in your local fish store, but it will probably be cheaper if you order online.
But anyway, I think because your tank is only a week old, and you've got lots of fish already... you're bound to have some deaths happen. So hold off on buying any more fish. Instead invest your money in a test kit, thermometer like you said, and a syphon if you don't have one for water changes, and make a schedule for water changes. Perhaps also, set up a quarantine tank. Here's a link on the forums about cycling:
http://www.fishforum.com/freshwater-...ium-cycle-252/
Someone else wanna jump in here and give opinions? :3
Also, look in here for lots of useful info!
http://www.fishforum.com/freshwater-...nt-topics-257/