Make sure you plan out what you want this too look like. If you want a waterfall, make sure you drill a hole for the pump line or intake to go through.
Fire belly toads and newts do not get along, you'll end up with three legged newts.
As a general guide to making a vivarium: Clean out your tank pretty thouroughly using water, an aquarium safe soap, and a rag. Make sure everything is completly dry and silicon on a glass / plexiglass / acrylic shelf to the tank. Your shelf can be any length, width, or height so long as it is a) angled slightly downward so water can drain into the aquarium b) if you have a curved lip on your shelf you'll need to drill/cut holes for water to drain from and c) your newts will need easy access to the water in the way of a beach or ramp.
After the silicon cures (two days) you can lay down substrate, plants, rock, and drift wood. Personally, I'd moisten it before placing it on the shelf, especially if you have drainage holes. Use an aquarium planting substrate on the shelf as the mixture of chemicals for outdoor gardening contains chemicals toxic to fish.
Your firebelly newts won't like or require anything more than room temperature (70 - 75 degress in the summer) and a little cooler in the winter, so a heater for the water isn't necessary. However, coldwater really limit the selection you stock with. You can use a heater, but keep the temperature around 72-74, this will allow you to keep platies and a few other hardy tropical fish with relative success. I wouldn't reccomend the betta as they like upper 70's to 80's.
You will also need a few areas of different mostures. The newts won't want to always be wet so they'll need a dry place to hang out in/on/at.