Firstly, with new tank and filter filled with dechlorinated tap water, “fishless cyle” them for ~6 weeks (until zero reading for ammonia and nitrite), then a “qualifying week,” then buy fish.
“Fish-in cycling” is cruel and for a responsible pet owner, requires a lot more work, you need to keep the water safe for the fish at all costs… Which could easily mean doing ~50-95% water changes every day for ~13 weeks.
Fast growing plants, such as Duckweed, can help keep the water safe for the fish. They will use ammonia/nitrite/nitrate as a food source, providing they are given a suitable light period of ~6 hours per day and other micro/macro nutrients.
Some dechlorinator, such as Seachem Safe and Seachem Prime, will temporarily mop up some ammonia/nitrite/nitrate for up to ~24 hours, if up to 5x the standard dose is used.
The general rule is that a fish requires at least 6x its adult length and 2x its adult width as footprint dimensions for the tank. However, some species are extremely sedate (such as my Chrysichthys ornatus), while others are hyper-active (such as my last remaining Brachydanio rerio… which I would not put in a tank shorter than 120cm long).
Smaller tanks can quickly become toxic soup coffins in an immature setup, get the biggest tank you can accommodate, a 120x30x30cm would be great as a startup tank for smaller fish that reach <15cm.
~33-50% water changes per week if their are no toxin readings.
Research what fish are safe to combine! Some are territorial like my Steatocranus casuarius cichlids; some are pig-like eaters that could starve reserved eaters to death such as my Illyodon xantusi; some like a tranquil water flow such as my Pantodon buchholzi; some need lots of water flow like my Chaetostoma catfish; some are vegetarian like my wood eating L204; some are opportunistic predators like my Chrysichthys ornatus and Ctenopoma acutirostre; some require big social groups of their own kind like my 25 Pareutropius mandevillei etc.