Some of the answers depend on what you want to do with your nano tank.
My overall recommendation is to keep it simple.
You might want to check out this thread https://www.plantedtank.net/forums/10...-examples.html
for pictures and ideas.
- Yes, CO2 is for plants and I would stay away from it, given the size of your tank.
- Use any "gravel" (substrate) you like. If you have an existing tank, use some substrate from it. Otherwise, anything from sand, to pebbles, to lava rock, to specialized aquarium substrate will work.
- Plants, whatever you have handy. Moss, Java Fern, Blyxa Japonica, Rotalas, mini Pennyworth, Ricca, and a lot more. Do consider using floating plants like Frog Bit, Water Lettice and such. Stay away from duck weed.
- I would go without heater, especially if your room temperature stays above 64F or so.
- I would avoid putting any fish in a tank that small. Go with shrimp and / or snails. If you must have fish, check out Endlers and guppies.
Trial and error are the best teachers.
If you are serious about having your nano tank healthy and running for years with little maintenance invest your time in reading the forums and learning as much as you can. Above all, enjoy!