What are some good beginner plants for a houseplant terrarium?
This does depend on a number of things, not the least of which are variables like the size of your terr, the conditions you will be able to provide, and the "look" you want. Many folks, not surprisingly, want at least a few plants that will bloom. Keep in mind that such plants often will need quite a bit of light. From the foliage aspect, there are a number of neat plants that can provide you with different textures/looks.
A few plant examples:
African violets [note: while growing AV's in a terr is almost effortless, some people do find it difficult to get them to bloom]
small "northern pines" are slow growers
small variegated ivy