I would get a test kit. A good one is API freshwater master test kit. It covers what you need (pH, ammonia, nitrite, nitrate), is fairly easy to use, and accurate enough for what you need it to do.
What do you know about the nitrogen cycle and do you know how to cycle a tank? You need to cycle the tank.
The rams gasping could be due to poor water conditions or lack of O2
. Did things improve at all (even if temporarily) after your water change? Or you could have just got fish that where already sick or maybe they werent properly acclimated before adding them to your tank.
Im not convinced about the compatability of the fish selected by the fish store, especially temperature. 26c is on the warm side for the tetras and cold side for the ram. Its probably the best compromise temperature though.
Honestly, if a store irresponsibly sold me fish the way that happened to your mother and they wouldnt take them back, i would just bag them up, return them, and leave them on the counter, and write off the cost. That's probably not your thought though.
Until you can get a test kit, change water daily, like 30%. When you can test your water, do a fish in cycle. Depending on what filter you have would consider adding an airstone to make sure your water is properly oxygenated.
You have a lot of fish to try and do a fishless cycle, so i foresee a lot of water changes in the short term.