Sorry, I don't mean to sound offensive, but have you done
any searches on this forum? There's plenty of information out there, and I think that if you look around, you'll truly be inspired. This is a great place just to browse and learn, and you can find what
you want to do with your aquarium, instead of what everyone else tells you to do. That's one of the joys of aquascaping- expressing yourself through your designs. It's so much more fulfilling when you do your own thing and love the results. Just some personal thoughts.
Take a tour through the photo gallery and find what inspires you, then cater to what you see in your head. Personally, before you even get your first plant, I would read Rex Grigg's guide- you can find it here:
http://rexgrigg.com/
To begin with, what type of 20g is it? A high, an x-high, or a long? Also, are you just using the standard 15w bulb that came with it, or are you using an alternate light source? About how much gravel do you have in the tank, and what themes do you want to portray?
I imagine you have low lighting, so to start off with, look into mosses, along with plants in the genus: anubias, cryptocoryns, bulbitis, bacopa, hygrophila, microsorum, and ceratophyllym. You might want to try some manzanita driftwood in a lattice, as low-tech aquaria often use these to great effect, though I have seen rockwork used very well too. A great resource for plant information can be found at aquaticplantcentral's plant encyclopedia:
http://www.aquaticplantcentral.com/forumapc/plantfinder/.
As for fish, you will want to go with smaller species, as it helps to make your aquarium look larger. If you want fish that produce little waste, look into white clouds (
Tanichthys albonubes,) zebra dannios (
Danio rerio,) and similar species; however, I again strongly suggest that you look around, see what you like, and then make your decisions.
Lastly, welcome to the board! It's always nice to see new faces, and I hope we prove helpful to you. Good luck!!