hi ben
Chameleons are harder to keep than most geckos, that tank size is perfect for a gecko, but chameleons need height more than anything and theyre humidity requirements are quite demanding. There are a few species of gecko you could get to start off with, ideally the leopard gecko like ours is the most popular and reccomended, and unlike other geckos do not climb walls as they dont have the powerful suction on their feet.
There are many good books available on the care of leos, heat is the main factor usually supplied via heat bulb over the top or a heat matt underneath, or both as some people do, as at some point the bulb will have to come off for night time, make sure all heating is statted on a good thermostat, and remember leos are nocturnal so they will only really venture out in the evening, their native country is parts of india, pakistan and afghanistan so gives you an idea of temps.
They feed on crickets, locusts, mealworms and waxworms etc, meaning theyre insectivorious.
Ours will feed solely on waxworms and despite claims that this is too fattening for the leo, it hasnt done ours any harm for the 5 odd years shes been alive.
Provide plenty of hides, ideally sand as a sunstrate and a nice shallow water dish, thats kept clean, remove any mess matter and soon you will have a friend for life. You could even get a pair, but make sure you keep to the male and female rule or 2 females, not 2 males.
If you need any help, just give us a shout and il be able to help you more.
Luke