Friendly advice to not get fish unless you're ready for a somewhat pricey hobby.
Not only that, but there's a lot of misinformation out there about aquarium fish. Make sure to do your research before purchasing any living thing (I'm including plants here tbh, but that's just me i know).
So if you're thinking about getting that betta in the cup because he looks sad, here are a few tips to get you started right:
Do research on the species you're considering BEFORE you make your purchase. 90% of aquarium keeping is getting things set up in advance so as to not stress the fish (or you) out
Never forget to cycle your tank first. There are two ways to do it: fish-in cycling, and fishless cycling. The fishless route is considered more humane, as fish are not exposed to toxic ammonia while the bacteria colony develops. It also increases your chances of fish surviving the move to a new tank.
Live plants may seem intimidating, but they are a got damn blessing. They help eat up extra ammonia and other toxic chemicals so your fish stay comfy, some species will help oxygenate your water which means you can fit more creatures (not necessarily fish, but we'll get to that), give a ton of hiding spots to help reduce aggression and injuries from tank mates, and just typically look nicer than fake plants, tho silk plants do have a nice, almost natural look to them.
Always under populate your tank at first. Even if you've cycled your tank before getting fish, there's still going to be a mini cycle when you add any number of fish. Depending on the bio-load of the fish, you'll want to start with just two or three and add a couple more each week until you have the desired stock.
Larger tanks are technically easier to maintain, but be prepared to make large water changes. We got a 20g long thinking changing 5 gallons a week wouldn't be much hassle. It's not a lot of work, but it gets tiring running back and forth between the kitchen sink and the tank. Our 10g standard is much easier, only needing a couple gallons changed a week.
The fewer fish you have to care for, the easier it will be to keep them healthy. We started off with a small school of guppies, a few amano shrimp, and a couple snails. We constantly had to check if one treatment for the fish was safe for the invertebrates and vise versa. Plus, it will make it easier to spot injury or disease.
Which brings me to my next tip, learn to recognize healthy stock when purchasing for your aquarium. It will save you a world of hurt, trust me. Helps to keep current stock healthy and saves you money amd work in the long run.
Always have a quarantine tank ready when adding new fish. Keep your new stock totally separate from your established aquarium for at least a week to make sure none of your new guys will bring in disease.
Acclimating is more than just sitting the bag in the water for temperature adjustment. This is the most common mistake I see new fish keepers make. Fish acclimate to temperature, yes, but also the ph level of the aquarium. One of the more common causes of death for new aquarium fish is ph shock. You want to add small amounts of tank water to the bag your new fish is in over 1-2 hours. You'll also want to drop the fish in with as little water from the pet store's tank as possible, to avoid contaminating your own tank with potential disease/parasites.
There's also no such thing as something that is totally self-maintaining. You will always need to do weekly water changes. Yes weekly. The once a month instructions you see on a lot of fish stuff is the bare minimum for a heavily planted, under populated tank.
And keep in mind, there are ALWAYS improvements to be made. If you get to a point where you feel you've learned everything there is to know on a subject and that you have all the answers, that's a sign you know basically nothing.