Yes, Hi trellis and welcome to the beginners section of TFF!
Andy is one of our best advisors and you've received some good comments up there! He's really good with stocking so I'm sure he'll check back in after you've got your tank all cycled and ready for more fish.
As said, you've missed a major topic of new aquarium startup. This is quite common and we deal with dozens of people in the same situation each month. Unfortunately, the hobbyist and typical tropical fish retail have gone down their separate paths over the years. As the world has grown faster and more impatient, the retailers have had to follow along and the slow puttering hobby of tank keeping has not been a thing their sales pitches could handle and still get sales. Meanwhile, the hobbyist side of things has continued the tradition of understanding this calm hobby and have added a few interesting bits of practical science along the way. And the topic at the center of all this is the "cycling" that Andy mentions.
The problem is the filter. When you buy one from the retailer, its really just a bunch of hardware and it needs to have a hobbyist that has learned about cycling be able to understand some things about it. It has three functions: mechanical filtration (catching debris,) chemical filtration (an optional thing we don't need at the moment,) and finally, "biological filtration," one of core bits of magic in the hobby and a crucial one for beginners. Our three articles in the Beginners Resource Center: The Nitrogen Cycle, The Fish-IN Cycle (the one you'll actually use for your actions,) and The Fishless Cycle (another one to add more perspective to your learning.)
You are unexpectedly in a Fish-In Cycling situation and are going to need to change lots of water (unless you can work out the return or temporary return (they will try like crazy to talk you out of it of course) of these first fish you have.) A water change (and they are probably urgent for you) must be done with good technique: A gravel cleaning siphon should be used to deep clean the gravel as the water is siphoned out (this helps greatly with a couple of the poisons you are removing.) Prior to getting a good test kit, you should change the maximum amount (85%, 75%?) that you can such that the fish are still barely submerged. The return water (tap water) should be treated with your conditioner (to remove chlorine and chloromines) at 1.5x to 2x the amount the instructions say. If you don't have conditioner, Seachem Prime is very concentrated (thus costs less) and is very good for beginning tanks. You must also roughly temperature match (your hand is good enough for this) the return water.
Second to the urgency of the first water changes is obtaining a good liquid-reagent based test kit. Most of us like and use the API Freshwater Master Test Kit, which has tests for ammonia, nitrite(NO2), pH and nitrate(NO3) in it. The test kit will save you from too many water changes and will help you determine your progress and when you've accomplished the goal of cycling.
Speaking of that goal: You have to be a bit of a detective and use your tests to figure out the percentage and frequency of water changes which will keep your water always at or below 0.25ppm ammonia or nitrite (same max number for either one, makes it simple) before you can be home again to test and potentially do another water change. With attention you can figure out as reasonable a pattern as accomplishes the need your fish have.
WHY? Why are the water changes needed? The reason is that you are having to be a "manual" biofilter for your fish until the two needed species of beneficial bacteria can grow in the biomedia portion of your filter. This takes typically one to two months. Nowadays we mostly do this with a fishless cycle but when necessary, a fish-in cycle can accomplish the same thing, its just a lot more work and more risk to your fish!
When fish move water through their gills, they give off ammonia. Fish waste, excess fish food and plant debris all break down into still more ammonia. In nature this ammonia is quickly diluted by millions of gallons of fresh water. In our tiny tanks it builds up quickly. Ammonia, even in tiny concentrations, causes permanent gill damage leading to shortened lives or death. The first species of bacteria we grow can "eat" this ammonia and turn it in to nitrite(NO2).
Nitrite(NO2), even in tiny amounts, causes suffocation and rapid permanent damage to nerves and the brain. The second species of beneficial bacteria we grow can "eat" this nitrite(NO2) and turn it in to nitrate(NO3). Nitrate(NO3) is not nearly as deadly a problem and can be kept at a reasonable concentration in our tanks by weekly water changes.
Once we have grown two fully funtioning and robust colonies of the two types of beneficial bacteria in our filters, we say the filters are "cycled" (they are now carrying out the "Nitrogen Cycle" of environmental science) and we can retire our emergency maintenance and begin normal tank maintenance.
Hope my paragraphs will give you some more material to look back at as you are learning this new skill. It often feels a bit complicated to newcomers, but don't worry, we've all learned it and its easy! Its the key to great water and tropical fish are all about keeping great water!
~~waterdrop~~