The best small monitor to start out with are either yellow or red ackies, they are very active and are the easiest to care for. They can live in a 4x2x3 with a sold top +foot of dirt and around a 130 to 140f basking spot and mostly eat insects. Ackies will come right up to you there not shy at all and average 15 to 24 inches.
Def. stay away, tree monitors are best left to more experienced keepers, they require alot more then most monitors. They get in the 3-4ft and are the largest of the tree monitor species, they need tons of hides, high humidity, and need to settle in to captivity as 99.9% are WC. Some people say there's are CBB ask for proof (egg pics,breeding,hatching). They need a extremely tall cage in the range of 8ft tall as they feel safer/better over your headHow big do the blue tree monitors get? What size cage requirements? Aggresive? They're pretty
Whats the largest cage you can have?
Bryan