Hey Merissa welcome to the chameleon forums.I have to agree with Lisa and Jaxy,pygmies are the only choice you have for a 12x12x18 exo terra glass terrarium.Now pygmies can be very difficult to keep for a first time chameleon keeper,so make sure you know how to care for them before you get started.In your case i recommend that you start with a RIEPPELEON BREVICAUDATUS(BEARDED PYGMY CHAMELEON)they are very hardy compare to other pygmies and they also eat a wider variety of common feeders.Try to find some captive born specimens,the big advantage is knowing the age of the chameleon you want to purchase it is rather important since they dont live much more then 3 years.RIEPPELEON BREVICAUDATUS prefers a temperature between 70-80F,you have to create a heat gradient for the chameleons thermal regulation,the top of your cage near the lights should be around 77-78F while the bottom at ground level should be around 70-72F with a slight drop of temperature at night 62-65F,you should really try and avoid temperature warmer then 80F during the day and under 60F at night to prevent health issues.Mist your cage 2-3 times a day,make sure your chameleons get a chance to clean their eyes and drink,keep humidity around 70% with a spike of humidity after misting.Please keep in mind that pygmies are not recommended for first time chameleon keeper.