I'm really sorry that you have just started fishkeeping and you're having all this bad luck. Please don't be put off the hobby.
Do you know what 'cycling' a tank means?
If not, click this link: http
/www.fishforums.net/index.php?/topic/277264-beginners-resource-center/ and have a good read, I guarantee it will help you find out what is happening with your tank and answer some questions you may have.
PS. I have been fish keeping for 7 years, and I cannot manage to keep guppies alive for longer than a month, trust me, i've tried. I keep platies mostly, they are very playful, hardy and they are gorgeous fish! So easy to breed.
Oh and one more thing! People at pet stores will say
anything if it will make you buy fish from them. If you have any questions about fish, never ask people at pet stores, it's all rubbish... Read things online and come to this site and ask questions.