If you want the theme from nemo you will need to get an ocean. The only fish that will work in a 20 gallon is a clown fish (Nemo) and a royal gamma (Gurgle). Yes I did my research to figure out the fish names.
Fake decor is fine but you will want some live rock as well. The filter from a freshwater is fine personally I would get an aquaclear 110 and turn it into a fuge. The more water flow the better so you will probably one at least one power head a koralia is the type i use. The lights you will want at least a 10K. Aragonite sand for the substrate.
A refractometer, liquid test kit for ammonia, nitrates, and nitrites and a good source for RO
That is what you will need for saltwater.
For fresh water you can get by with an aquaclear filter rated for a 30 gallon, heater, light, liquid test kit and prime.
For fresh water get prime to treat the water.
Before starting an aquarium research and read about cycling it is basically the same for fresh and saltwater.
The biggest mistake people make when starting a new tank for either salt or fresh is to rush. Take your time do it right so you aren't figuring what you have to do to fix the mistake you make.