Ok, we'll start slow. For now you can just run to the store to get your water tested, most pet stores will do it for free. But if you're going to get into the hobby (and you care about killing fish, which clearly you do) it's probably best to invest in a water test kit. API makes a good one. Get the liquid, not the strips. The strips are hard to read and often aren't as accurate. What you need to know is how much ammonia, nitrite and nitrate is in your tank. If the ammonia is over 1 ppm, do a water change (with a dechlorinator, Prime is the most often recommended) to get it below 1 ppm. Same thing with nitrite, if it's over 1 ppm do a water change to get it below. Read the stickys on cycling, or go to MissFishys profile and click on the link to her website to find info on cycling. At this point, you're doing a fish in cycle, which means that unless you keep an eye on the water parameters (ammonia, nitrite, nitrate) and do water changes to keep the levels low, you will probably lose more fish. Hope this helps. Let us know if you have any more questions.