Glad you came here for some help!
This is the first step imho... getting to know some basics.
Guide to Starting a Freshwater Aquarium - Aquarium Advice
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You can go to the top green bar where it says articles
freshwater
articles
Look around for what topics seem to interest you.
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As for tank size, fish and other information...
My recommendations are always to take a bunch of field trips to lfs's (local fish stores) especially privately owned stores.
Spend some time looking and not buying fish.
Watch the display tanks. Looks at the tanks of fish for sale.
Start making notes of the fish which capture your attention, Latin names are useful later in referencing, and what you like about them. A pic or 2 can help as well.
After a bit of time you will likely start to notice something(s) which you are most drawn to.
The #1 or 2 fish/creatures on the list will then be the center of the build for your tank.
If you fall in love with an Oscar fish you would need a pretty large tank and need to know that often these guys do not like plants and other tank mates.
If you fall in love with dwarf shrimp, you can't have a tank with a big Oscar, cause he's eat them. Etc.
Anyway, your fish choice might need a large school, a certain temperature range, or other special requirements.
Some fish you buy at the lfs could grow to a foot or larger. Redtail Catfish (5 FEET, lol)
Fancy Goldfish need a very large tank compared to what you might first think and like to eat a good many kinds of plants. So if you really love and want a nature style planted tank, then the GF might not be ideal.
You may find some things do not match up. And an alternate option would be in order.
Once you find out more information about the fish you like best, then you can build the tank around the fish.
Learning more about other matching types of fish which WILL work with your choice.