Hi Pammy welcome to Fish Lore

I'm really sorry about your guppy and the problems you're having with your tank.
I apologize in advance, this is going to be a long post.
It's hard to start out and be given the wrong information.
I'm going to give you some links to read to give you a better understanding of what goes on with a new tank and some recommendations.
The first thing to consider is your tank is over stocked. When starting out with a small tank (and small fish) a good guideline to go by is 1" of adult fish per gallon of water.
This is by no means a hard rule as you'll learn along the way.
Fish create ammonia through their waste. A small tank can only handle a certain amount.
When you're overstocked they made too much waste for the amount of water in the tank.
It would be a good idea to consider returning some of your fish or if you want to keep all your fish, you'll need a bigger tank.
The guppys and the neons fully stock you tank.

The shrimp can stay, they don't create much waste.
This link will help you learn about the
nitrogen cycle
Since you're cycling with fish, it would be a good idea to do 50% daily water changes and use Prime as your water conditioner. This will detox the ammonia for 24 hours between water changes.
Your tank will be cycled when you have readings of 0 ammonia, 0 nitrites with some nitrates showing.
Be patient, it can take up to a month to cycle a tank.
It would be a good idea to get your own water testing kit. The liquid test kits are much more accurate than the strips. Many of us use the
API liquid
test kit, it's very reliable.
Test every day during the cycling process and keep a record of it. This way you'll know how the cycle is progressing.
If the white spots look like salt it's probably ich. This can be treated with out meds.
Slowly raise the temperature to about 84F, keep it there for at least 2 weeks, increase the aeration with an airstone because there's less oxygen in warmer water.
Do thorough gravel vacuums to pick up any ich spores in the gravel.
It's possible they came from the store that way but also ammonia is toxic to your fish. It weakens their immune system and makes them more susceptible to disease.
Here are some beginner articles to read:
Beginners
I'm sorry for the information overload. I know it can be very confusing when first starting out.
Hang in there, it gets better after the tank is cycled
