I want to make it as natural looking as possible. What is the best sand to use?
If you want it natural looking, you may opt for something like river rocks, but if you are planting I suggest going with ecocomplete, it's a good planted tank substrate for beginners.
White sand, like most PFS or playsand, really isn't the most natural look for most fish. It also has a tendency to wash out colors on fish, while darker substrates bring the colors out. It's really cheap, though, so it is a staple in many aquariums.
I do suggest a nutrient enriched substrate though if you plan to go planted at any point. Ecocomplete and floramax are some readily available options that are simple enough to set up.
One method that people often go with also is a compromise of the two, and layering soil underneath sand, but I'd save that one for a future project.
What are some good beginner plants?
There's a ton of different low light plants that will work, but as mentioned above, it depends completely on your lighting as to what you will have success with. There are a few that work in pretty much any light, java ferns, anubias, java moss are all good. Slightly more demanding are the wisteria, hornwort, crypts, different ludwigia species, valisnera, swords, among several others.
I can't really recommend any particular filtration, I use sponge filters on most of my tanks.
Best of luck in your new tank.