Yeah, hopefully you bought one 20+ gallons in size. Those are easier to take care of than something under 20 gallons. Anyway, there is some good stuff online, but if you aren't a fan of staring at your computer screen for help all day then go to Petsmart, or Petco. They have book racks with lots of good sources.
A few quick words of advice...
1) Canister filters are quieter than HOB filters, or Sponge filters run by air pumps. Get a canister if you can afford it, so you don't go insane while laying in your bed, or sitting in your chair.
2) Live plant your tank right away. I was scared to do this and bought some decorations only to take them out a few months later and spend more money live planting it. Java Ferns, Amazon Swords, and many other plants will do fine with the standard LED lights that come with most tank lids. Add Flourish to keep your plants healthy.
Research the breeds of fish you are interested in. Some are bottom swimmers, some are middle swimmers, and some are top swimmers. All fish also have a minimum tank size requirement and are either considered aggressive, semi-aggressive, or community. It is best to have some top, middle, and bottom swimmers to make the tank look full.
For example I currently have a 20 gallon with a lid with top swimming hatchet fish, mid swimming long finned skirt tetras, bottom swimming cory catfish, and a mystery snail. Those are all considered community fish and can live together. It is planted with java ferns and Amazon Swords. That works.
What wouldn't work would be a lidless 20 gallon with Hatchet Fish (they jump out of the water), a betta (aggressive and have to be kept alone), a bala shark (need a much larger tank), a school of Tiger Barbs (semi-aggressive fin nippers), and 7 cory catfish. That would be an overstocked tank that is undersized for some of those fish, and is also a mix of fish that aren't compatible with each other.
These are just things you need to think about when setting up your aquarium.
Also I would like to add, I have read on here that Goldfish need at least a 30 gallon tank and a general rule is 20 gallons for the first one and 10 gallons for each one added after that. When I set mine up I was also told to do either a goldfish tank, or non-goldfish tank because it is tough to keep the fish alive when keeping non-goldfish with goldfish. Plus goldfish like colder water than tropical fish.
Step 1: Goldfish, or non-goldfish tank?
Step 2: If you selected non-Goldfish (Freshwater Tropical) then start doing your research.
Don't add fish for a long time(month or so, it would help to get the API Master Freshwater Test Kit), so that the nitrogen cycle can be completed(I learned that the hard way). You can read up on it, but basically you are ready to have fish in your tank when your ammonia is basically 0, your nitrites are basically 0, and your nitrates are between 10-30 I'd say(it depends on the fish you want). In order to achieve these results you routinely change out the tank's water, which lowers ammonia and nitrite levels. Test with your test kit every few days, to make sure the process is going along well. If you have added a bacterial supplement or just some fish food to get the process going, you will begin with ammonia, which will then become nitrites, which then end up as nitrates. So you should see lots of ammonia in the beginning, but very little in the end of the cycle.