Agree with the advice from ZZ. Algae and overheating will be important considerations so keep thinking about and working on that aspect. The proper cycling of your filter (and thus your tank, as we sometimes say) is also of critical importance and as much for your learning as a family as it is for the first tank of fish! It is one of the bases of good fishkeeping knowledge.
In your case several major branches are still under discussion: you may end up fish-in cycling, or fishless cycling with mature media still in the filter or simply fishless cycling from scratch. Each of these three things is very different from the other at least in some ways. It is also important that you not misunderstand the time-frames involved. As ZZ said, you might be already cycled, IF you have living bacterial colonies still in the filter media. If the aquarium is established (say, 6 months old or older with owner not having changed filter media) and you get it all with fish or just after fish have been re-homed then you will be working with good bacteria and will only need to "qualify" them as having survived the transition. If fish are there then your test results over a week or so of ammonia and nitrite(NO2) will reveal how the filter is working. If there are not fish then you'll qualify the filter by subjecting it to simple household ammonia dosing, per the frequent discussions you'll begin noticing here on the beginners forum. If the filter somehow comes -without- mature media and you are starting from scratch and can not get mature media from another source then your idea of cycling time is incorrect, it can often take between a month and 70 days for a from-scratch fishless cycle to correctly complete.
One of the very first things you'll need with be a good liquid-reagent based testing kit. Salifert makes the very best if you can find those (they are a bit more complicated in that you buy individual kits for the things you need, which will be ammonia, nitrite(NO2), nitrate(NO3) and pH at a minimum. Many of us in the past have used the API Freshwater Master Test Kit and this has worked well. It's really the -knowledge- of the timing of things and little yes/no hints from the kit that are important and not so much the -values- that the kit is returning ultimately, but developing that skill does take a while and often requires sorting out help from the members.
Other basic but major skills are learning about the weekly substrate-clean-water-change and the maintenance of the filter system. Your idea to keep a log is important. I always keep simple spiral-bound aquarium notebooks with diaries (that are usually daily during the first months of a tank) and record virtually all that happens to the tank and my other thoughts and observations. I've found (assuming no house fire of course) this to be not subject to the whims of computers and forums etc. and I like the quiet non-computer feel of using my notebook during tank cleaning mornings.
When fish draw water through their gills, they release waste ammonia directly to the water. This is quite different from land-bound humans who need to retain waste for a while to extract the precious water. In the wild, millions of gallons of fresh water immediately dilute this ammonia and keep the fish safe from it's toxic effect. Ammonia, even in tiny concentrations, can quickly lead to permanent gill damage. The first species of bacteria in our biofilter (we like to call them the A-Bacs because they process ammonia) will convert ammonia (NH3) in to nitrite(NO2), which, unfortunately, is also a deadly toxic chemical to fish. Nitrite(NO2) can attach to fish red blood cells just like oxygen and when it does, can result in a process the destroys the red blood cell, suffocating the fish while causing permanent nerve and brain damage. The second set of bacteria (the N-Bacs) will process nitrite(NO2) into nitrate(NO3) which is also not great for our fish but is safe enough to let build up slowly and be controlled by our regular water changes and other maintenance of the tank.
This "Nitrogen Cycle" that I've briefly described is the beginning part of the kind of more complete understanding about "cycling" that we like all our beginners to work on as they come through the beginner section. It is not all about simply getting past that first cycling of the first filter. It is about gaining the kind of hands-on feel that prepares you to handle problems and emergencies in your future years as a household taking care of freshwater tanks.
Good luck and I hope you enjoy your stay at TFF and get off to a good start. The members are great and will take good care of you.
~~waterdrop~~
Amnesia - I was going to throw in my two pence worth to help, but then saw WD's comprehensive post.... lol
