Welcome fishfriend022 to the hobby and to AC! Sad to say, but in my experience most people who are selling animals don't seem to know a lot about them. Probably someone who just liked puppies or something and decided to start selling pets and supplies. They're not bad people, just not well informed. A few thoughts...
It is admirable that you want to start off right, and you've come to a good place. It really will help make things better for you in the long run. The work up front will make maintenance easier along the way and will make for happier and healthier fish. You've already gotten some good advice about doing that. My primary piece of advice is don't get too uptight or overwhelmed about it all. It's a hobby and it is supposed to be enjoyed. Apart from that, here a few things to help.
Figure out what size tank you want and start there. You'll need some sort of light, a filter that will handle your aquarium size, and a heater if you want to do tropical fish. I would personally recommend tropical if you're not going to have a very large aquarium. More options for you. If you want to stay small (5 or 10 gal), you might consider a single male betta (lots of colorful options) with some shrimp or snails, a small school of tetras (neons, cardinals, etc), or a few fancy guppies.
Get things set up, making sure your water is clean and properly conditioned (by this I mean use something like Seachem Prime or API Stress Coat to neutralize chemicals in the tap water), dial in a temperature that is good for fish you want (probably somewhere 76-80 degrees), and take the time to "cycle" the tank. The cycle post may be a bit complex, but what it comes down to is you need to build up some good bacteria in your tank that will help process the waste from your fish. Otherwise, the water gets toxic and your fish die. You can do this more quickly if you have a friend with a tank or the pet store can give you some filter media or gravel that is in a well cycled tank. You'll basically be bringing bacteria in without having to build it up artificially in the absence of fish/feeding/etc. Once you've done that, add just a couple of fish at a time until you have what you want.
After that, you'll just have to keep up with the quality of your water. You'll want to do regular water changes. Don't empty the tank and scrub it! Just use a cup or hose to take some water out and put clean water back in. Make sure that water is close temperature to what you've taken out, and add more conditioner with it to neutralize those chemicals. A good test kit will help. Also, don't overfeed. This will cause water quality problems and also help algae grow.
That may seem like a lot, but it really is pretty simple. Just take it step by step and don't be afraid to ask questions. You might get an occasional jerk on here who wants to make you feel stupid, but they try to police that kind of stuff. Vast majority of us are happy to answer questions. We all learn together. Best of luck and look forward to hearing more from you!