Substrate is just whatever you use for the bottom of your tank, it can be gravel or sand, of any type/size. For instance, I have sand in my tanks, so I have a sand substrate. If you have gravel, you have a gravel substrate. It's just a general term.
I have a sponge filter on my 10 gallon planted sand substrate tanks and it works just fine.
Most plants will do fine, though I would stay away from any sword plants as they can grow quite large.
Java Fern and Anubias varieties are all great beginner low-light plants. They can be planted, but google 'how to plant' them first, as if you bury the rhizome they can die. It's easier to just attach them to driftwood/rocks.
Wisteria, Crypts, and Watersprite are also pretty easy plants.
May I ask what lighting you have? Plants need a light around 6500k to grow best. You may also want to look into getting a fertilizer, I recommend 'Seachem Flourish Comprehensive Plant Supplement'.
Good luck :)