Your 12" x 24" terrarium is fine for one leopard gecko.
Your UTH taking up 1/3 of the floor space is fine as well.
Having one humid hide and one dry hide is ideal. It can be as simple as two tupperware containers with a hole cut in the lid. One should have about an inch of damp coconut husk fiber in it and the other can be empty.
You will need three small bowls, one for mealworms, one for water, and one for supplement powder.
Leopard geckos are super easy to keep. Don't make it too complicated.