Hiya,
Sadly there are a lot of shops that aren't aware of something called "
Fishless Cycling". As GillesF says basically it's a process where you build up beneficial bacteria which turns ammonia (which comes from fish waste, aka poo and wee) into nitrite and then nitrate. There are quite a few stores that won't sell you a tank and fish in a single purchase for this very reason.
I've got a few guppies as my other half loves them (real little characters) but they're actually quite weak little fish, very prone to disease. If you're planning on having males and females then you need twice as many females as you have males as the males *will* hassle the ladies for some guppy loving - this is possibly what happened as well as there being an ammonia problem. Also be prepared for a lot of young as guppies tend to reproduce as often as they possibly can! I only have males in my tank as I don't want any young uns!
Personally as you still have a couple of guppies left, I'd grab an ammonia and nitrite test kit. This will really be the only time you need a test kit as they're pretty unreliable
😉 Check your ammonia level every evening and then see if you need to do a water change. Ammonia is pretty toxic to fish, but you need enough in the tank to encourage the bacteria to grow but balance that against the fact that too much will kill your fish! I'd say once your test kit hits 1~2ppm ammonia do a 20% water change. Over time your ammonia will naturally lower as those bacteria grow. When it reliably hits 0ppm start to check your nitrites - use the same principal as the ammonia. Once that's reliably 0ppm you can put in a few more fish.
It could well take a few weeks to get things cycled and happy. Be patient, it's worth it in the end, use the time to read through the forum, work out what plants you'd like, find some nice bog wood...
pete
This is obviously my opinion and there are a lot of different methods of cycling - however I have always been told and read that you shouldn't put fish into a tank where there is ammonia or nitrite present.