Chameleons, while fun to watch, have fairly complicated care requirements, such as high humidity and like... dripping water dish or a daily misting, since they don't tend to drink from water bowls. If you do decide to get a chameleon, I'd recommend a veiled chameleon or a panther chameleon (more expensive, but also more beautiful in my opinion.) Also, chameleons require a well ventilated cage, so, an all-screened cage would be appropriate, not a glass tank, and they require fairly large cages.
For that small of a tank, maybe consider getting a pair of leopard geckos, they make good beginner pets and are nocturnal, so they don't require the uv lighting. Another choice is the crested gecko, but generally try to stay away from day (diurnal) geckos as your first reptile unless you do a lot of research (which you should do with any pet anyways.)
Have you considered a snake? Maybe a cornsnake is an option because they don't need to eat as often as a lizard.
Maybe you should consider going to a local reptile show, there you can talk to the breeders and learn all about an animal that you might be interested in!