welcome to the forum! For a 12x12x18 would be perfect for two, maybe three
arboreal darts like bumblebee dart frogs or Dendrobates leucomelas. They are great beginner darts.

if you want a very naturalistic terrarium a custom backround of great stuff foam can be made just by spraying it on the back with branches and pieces of wood or cork rounds for plant pots. if you do plan to make the custom backround, make sure to leave two to three inches of space for a drainage layer for plants. once the great stuff dries (and remember the foam will expand)you can use 100% silicon to cover the great stuff. if the silicon gets on the glass, you can always get a razor blade and gently scrape it off. after the silicon has been spread across the backround. cover with dry eco earth and wait for one day to dry.

Once this has dried, you can place the drainage layer of hydroton in the bottom of the terrarium, this is so the terrarium dosent stink and so if you overwater the plants, there will be extra water for keeping substrate moist and providing a place for beneficial bacteria. Its adviseable to use window screening or weed block to separate the drainage layer from the substrate (cocoa fiber or tree fern panel) the bedding can be two to three inches deep. At this time you can add in Springtails if you would like to make cleaning the terrarium easier, these guys eat the mold and frog poop within the terrarium.

This is by far the best part of building the terrarium, planting. A large variety of plants can be added to a terrarium of that size such as small bromeleads, wandering jew, star plants and many more plants for dart frogs, these can be added in the cork hollows and in the substrate, Java moss and zoo med frog moss can be laced on pieces of driftwood, hides and by small bodies of water. Im not sure how to add a waterfall into the terrarium by using the great stuff method though. After planting is finished you can use a light on top of the terrarium

I hope this comment has answered your questions