Look around on the net for places that have temp/humidity/light similiar to what you can provide, then pick plants from that region, or just ask around at your local home depot/plant spot as Im sure they will be helpful, I have a few earth stars (3) and alocasias (3?) that will do well if you give them good light and humidity for the alocasia, you can have them for cash or trade, let me know if your interested, this site is kinda slow at answering questions but will do so.....eventually =)

Forgot to mention some brom's (3) that will do well if you give them good light, broms and earth stars do well when planted as epis, up high and watered well only occasionally, as stated in my last post, let me know if your interested =)

I would definitely add some lighting. Two full-length fluorescents should be alright. The brighter the better, to a point. If you get the lighting worked out, probably the easiest plants you'll find are smaller philodendrons, pothos ivy, creeping fig, and small bromeliads. Most of these can easily be found at your local home depot, and should be fine. Once the creeping fig gets going, it really goes, and is nice if you're looking for something to quickly fill in features of the terrarium.

Just wanted to let you know that, although dravenxavier is right, these are kinda good starter plants, pothos, most if not all ivy, and creeping fig will very quickly take over your entire tank, a 20G is a somewhat smaller tank so I would stick to smaller plants, the plants mentioned will very quickly block light to other plants you may have as they grow fast and "out of control" I tried to keep these when I started and it was a constant job trimming them back and keeping them the size I wanted as you trim one spot and four others pop up in another spot! If you dont mind a crowded tank then these plants are for you =)