Well, how much room do you have for a monitor?

A great beginner species is varanus acanthurus (ackie monitor) it's a dwarf monitor species from Australia. CBB babies are readily available and they're not terribly expensive ($200 or therabouts) they tame down easily, and grow from 2-3'. They can also live communally. A 30 gallon tank is adequate for one ackie.

Savannah monitors are the most common in the pet trade.. most are wild caught or farm raised and will need to be treated for parasites. Many tame down, but some don't.. a bite from a 4' monitor is not to be taken lightly. You'll need at least a 6' long cage when it's full grown.. monitors also need deep cages because they like to dig and burrow in the dirt.

I would NOT reccomend a water monitor.. they get HUGE and need a lot of room.. not to mention are very expensive to feed.