The footprint is 48 x 12.5, from your first post, same as the surface area.
Do you already have a filter that may have come with your tank? I like Aqua Clear filters & I also like 2 on most tanks (eventually for you but not right away). I would get an AC 70.
Substrate: very small gravel (like peppercorn size) or large sand (like pool filter sand not play sand) or maybe Oil-Dri if you want more plant choices eventually (some don't need substrate at all). All those are inexpensive options, the are many others. I prefer a dark color, white is hard to keep clean looking.
You wise to look at fish options BEFORE buying any. You also need to read up on cycling your tank, it's much easier on fish to do ammonia cycling ahead of getting any fish but it's hard to be patient. There's a "sticky" at the top of the newbie forum that explains cycling, read it!
For an easy coolish water tank (you may not need a heater) look at roseline sharks, say 5 or 6 (p. dennisonii), while cloud mountain minnows (10 or 12) & corydoras catfish (8-12) all the same species. In general schooling or shoaling fish are best in groups of at least 6.
Another warmer temp group of fish might be an Asian tank of harlequin rasboras (12-15), honey gourami (1 male & 1-3 females) & 8-12 kuhli loaches OR dwarf chain loaches (a. sidthimunki).
These are just suggestions, if you fall in love with cardinal tetras, dwarf cichlids or whatever, you have lots of options. Keep looking & researching...& asking us. It's best to keep fish with similar requirements, although there's room for some variation in temp or pH. Also look at adult sizes not the juveniles usually offered for sale. In 6 months or so most will get to adult size with good food & weekly 40-50% water changes.
