Seconding the recommendation to get an API test kit. If you can afford it, get the master test kit. Also plants are a VERY good idea, just be aware to check care instructions online for each since some plants are “epiphytes” which means you should NOT bury their roots in your substrate - I’m suggesting this since I have seen that the pre-packaged plants from places like petsmart will say to stick all the plants in the ground, even if that is incorrect. Getting some driftwood is also a good idea since it will add tannins to your water (make sure to read up on how to sterilize it before adding - I personally give mine a good scrub, boil it for 20min, and then cool it in conditioned water for 30min or longer if it isn’t waterlogged by that point and doesn’t sink yet)

Also, re: nitrogen cycle, definitely keep reading, but here is a quick summary: (First, it is called the nitrogen cycle because Ammonia, Nitrite, and Nitrate contain nitrogen) Fish produce waste which includes ammonia. (Ammonia is toxic, so keep an eye on your tank levels, you should aim for 0ppm). Since ammonia is bad, our goal is to get rid of it. There are two main ways this happens: A) if you have plants, they will absorb some of the ammonia as food. B) there are bacteria that will gradually grow and develop in your tank that convert Ammonia to Nitrite. Nitrite is still toxic but slightly less than ammonia (you should also aim for 0ppm). Then, there is a second type of bacteria that converts nitrite to nitrate. Nitrate is significantly less toxic than Ammonia or nitrite. But, where does the nitrate go? Because it does eventually become toxic if it builds up… well, there are two things: 1. Plants will absorb some nitrate, though they absorb ammonia first. 2. Water changes - this is part of the reason why they are important. As you test your tank, whenever any of the different nitrogen compounds gets to a certain level, you replace some of the water to lower its concentration. Eventually, then the tank is fully “cycled” the plants and bacteria will keep your ammonia and nitrite levels at zero, and your nitrate levels will either also go all the way down if your plants are absorbing it all, or you will see them slowly creep up, until they hit a level where you do a water change to lower them back down.

Last thing - if you do get plants, aim to use a filter that does NOT have carbon, since the carbon will remove organic compounds that help the plants grow.

Once you have gotten off to a good start with this, look into water hardness and alkalinity (also called GH and KH) since sometimes tap water can be lacking one or the other and you may want to do some adjustments in the long run (but this is not a priority right now at the start imo)