I would recommend to do a LOT OF RESEARCH before you get fish
I'm new to this world to but I have learn so much about the species and I highly recommend to do a lot of research
There are fish that are more aggressive some are more chill,
Choose what you want on your tank and then research compatibility
If it's your first tank
Maybe start with some tetras, guppy, sword tail, along those lines,
they are pretty easy to keep and very chill & full of diff colours
Make sure you buy a decent filter
So if u have a 20gallon maybe get a filter for a 40 gallon tank
Recommend
Air pump
Drift wood
Real Plants
Last note
Try to buy "fry" (babies)
Don't buy the older fish they will die sooner
Hope it helps