Guppies can stay alive quite well actually, if they die it just means
a) you got bad stock
b) your setup needs to be improved.

If you buy from a shop that stocks a whole bunch of guppy in a tank, they could actually already be of weak condition. You can always expect some of those you purchase to die off pretty soon.

I haven't been keeping fish for a while, no idea where's good in PJ.

Just went to Pet Lover Center in Ikano, the guppy stock looks good. Fins are not worn and the amount of fish per tank looks alright.

Bear in mind that you should avoid mixing male + female guppies in a tank if possible, or if you do then make sure ratio is like 2:1 or something, 1:1 ratio is no good.

The males will always harass females for mating.