Well, to start, there are several species of blue tongues. Some care such as humidity will differ, but overall, they have similar care.
The minimum size enclosure for an adult is a 40 gallon breeder. But if you can go bigger, then do it. Substrate really depends on the species. Northerns typically don't need as much humidity as Indonesians, so a lot of people keep them on aspen. I only keep Irian Jayas which are found in parts of Indonesia, so they need higher humidity. Some good substrates for the more humid species are cypress mulch, sphagnum moss, peat moss, organic top soil, coco fibre, repti bark, or just plain ol' dirt (just make sure there are no chemicals, fertilizers, pesticides, etc. in it). There is a form of Tiliqua gigas gigas called Halmaheras, and they need even a bit higher than most Indonesian species. These guys also love to burrow, so you'll need to provide several inches of substrates. Leaf litter can also be added for them to hide under.
Many people have successfully kept these guys on a high-quality, grain-free dog food. My personal favourites are blue buffalo, Nature's Variety Instinct, and Merrick. If you go the homemade route, some good protein sources are roaches, superworms, hornworms, silkworms, quail eggs, earthworms, snails, etc. Some good veggies are collared greens, mustard greens, turnip greens, cactus pad, carrots, green beans, etc. They can be offered a small amount of fruit on occasion such as banana, blue berries, apple, grapes, etc. Here is a good food chart:
http://bluetongueskinks.net/foodchart.html Also make sure to supplement with appropriate amounts of calcium with d3.
You generally want a basking surface temperature of around 100. Besides a probe, the best way to measure the surface temperature is to use a temperature gun. Ambients should probably stay around low 70s to low or mid 80s. You can use both uth/heat tape or heat lamps. I personally prefer heat lamps and only use heat tape if using racks or tubs. If you do use uth/heat tape, please make sure you have it hooked up to a reliable thermostat. As for UVB, plenty of keepers have kept and bred them successfully without using UVB, but it still wouldn't hurt to use it if you'd like to.
Now, for choosing a blue tongue to buy. Australian blue tongues you see on the market outside of Australia are going to captive bred. The most cost friendly and most available of the Australians are Northern blue tongues. As for the Indonesian species, most of them you find will be wild caught. I'm not against wild caught, but if you have no experience with working with and rehabbing a wild caught animal, I would suggest not getting one as a first blue tongue. A lot of them come in with mites, and many are dehydrated and carry parasites, so the cheaper cost compared to the Northerns will soon add up with vet bills. There are some breeders of Indonesian species. Irian Jayas and meraukes are probably the most available captive bred Indos, but it is still uncommon to breed them. There are a few breeders of Indonesians, Tanimbars, and Kei Islands, but they are far and few. Then for the Halmahera typed Indonesians. As far as I'm aware, no one has been successful with producing them in captivity. If any are labeled as CB, it means they were born in captivity to a gravid wild caught mother, so they will not be true CBB since they weren't bred in captivity. Another thing to avoid are hybrids. Most of the community as a whole frowns upon mixing the blue tongue species since it muddies the bloodlines.
This is a really good forum for blue tongues:
http://www.bluetongueskinks.org/forum/index.php
And here is a really good facebook group that has some brilliant breeders on there:
https://www.facebook.com/groups/728417770538748/
Hopefully I didn't forget anything, but if I wrote something incorrectly or forgot something, I will try to fix it.
